Colorbars and legends

Proplot includes some useful changes to the matplotlib API that make working with colorbars and legends easier. Notable features include “inset” colorbars, “outer” legends, on-the-fly colorbars and legends, colorbars built from artists, and row-major and centered-row legends.

Outer and inset locations

Matplotlib supports drawing “inset” legends and “outer” colorbars using the loc and location keyword arguments. However, “outer” legends are only posssible using the somewhat opaque bbox_to_anchor keyword (see here) and “inset” colorbars are not possible without manually creating and positioning the associated axes. Proplot tries to improve this behavior:

  • proplot.axes.Axes.legend can draw both “inset” legends when you request an inset location (e.g., loc='upper right' or the shorthand loc='ur') and “outer” legends along a subplot edge when you request a side location (e.g., loc='right' or the shorthand loc='r'). If you draw multiple legends or colorbars on one side, they are “stacked” on top of each other. Unlike using bbox_to_anchor, the “outer” legend position is adjusted automatically when the tight layout algorithm is active.

  • Proplot adds the axes command proplot.axes.Axes.colorbar, analogous to proplot.axes.Axes.legend and equivalent to calling proplot.figure.Figure.colorbar with an ax keyword. colorbar can draw both “outer” colorbars when you request a side location (e.g., loc='right' or the shorthand loc='r') and “inset” colorbars when you request an inset location (e.g., loc='upper right' or the shorthand loc='ur'). Inset colorbars have optional background “frames” that can be configured with various colorbar keywords.

colorbar and legend also both accept space and pad keywords. space controls the absolute separation of the “outer” colorbar or legend from the parent subplot edge and pad controls the tight layout padding relative to the subplot’s tick and axis labels (or, for “inset” locations, the padding between the subplot edge and the inset frame). The below example shows a variety of arrangements of “outer” and “inset” colorbars and legends.


Unlike matplotlib, proplot adds “outer” colorbars and legends by allocating new rows and columns in the GridSpec rather than “stealing” space from the parent subplot (note that subsequently indexing the GridSpec will ignore the slots allocated for colorbars and legends). This approach means that “outer” colorbars and legends do not affect subplot aspect ratios and do not affect subplot spacing, which lets proplot avoid relying on complicated “constrained layout” algorithms and tends to improve the appearance of figures with even the most complex arrangements of subplots, colorbars, and legends.

import proplot as pplt
import numpy as np
state = np.random.RandomState(51423)
fig = pplt.figure(share=False, refwidth=2.3)

# Colorbars
ax = fig.subplot(121, title='Axes colorbars')
data = state.rand(10, 10)
m = ax.heatmap(data, cmap='dusk')
ax.colorbar(m, loc='r')
ax.colorbar(m, loc='t')  # title is automatically adjusted
ax.colorbar(m, loc='ll', label='colorbar label')  # inset colorbar demonstration

# Legends
ax = fig.subplot(122, title='Axes legends', titlepad='0em')
data = (state.rand(10, 5) - 0.5).cumsum(axis=0)
hs = ax.plot(data, lw=3, cycle='ggplot', labels=list('abcde'))
ax.legend(loc='ll', label='legend label')  # automatically infer handles and labels
ax.legend(hs, loc='t', ncols=5, frame=False)  # automatically infer labels from handles
ax.legend(hs, list('jklmn'), loc='r', ncols=1, frame=False)  # manually override labels
    abc=True, xlabel='xlabel', ylabel='ylabel',
    suptitle='Colorbar and legend location demo'

On-the-fly colorbars and legends

In proplot, you can add colorbars and legends on-the-fly by supplying keyword arguments to various PlotAxes commands. To plot data and draw a colorbar or legend in one go, pass a location (e.g., colorbar='r' or legend='b') to the plotting command (e.g., plot or contour). To pass keyword arguments to the colorbar and legend commands, use the legend_kw and colorbar_kw arguments (e.g., legend_kw={'ncol': 3}). Note that colorbar can also build colorbars from lists of arbitrary matplotlib artists, for example the lines generated by plot or line (see below).


Specifying the same colorbar location with multiple plotting calls will have a different effect depending on the plotting command. For 1D commands, this will add each item to a “queue” used to build colorbars from a list of artists. For 2D commands, this will “stack” colorbars in outer locations, or replace existing colorbars in inset locations. By contrast, specifying the same legend location will always add items to the same legend rather than creating “stacks”.

import proplot as pplt
labels = list('xyzpq')
state = np.random.RandomState(51423)
fig = pplt.figure(share=0, refwidth=2.3, suptitle='On-the-fly colorbar and legend demo')

# Legends
data = (state.rand(30, 10) - 0.5).cumsum(axis=0)
ax = fig.subplot(121, title='On-the-fly legend')
ax.plot(  # add all at once
    data[:, :5], lw=2, cycle='Reds1', cycle_kw={'ls': ('-', '--'), 'left': 0.1},
    labels=labels, legend='b', legend_kw={'title': 'legend title'}
for i in range(5):
    ax.plot(  # add one-by-one
        data[:, 5 + i], label=labels[i], linewidth=2,
        cycle='Blues1', cycle_kw={'N': 5, 'ls': ('-', '--'), 'left': 0.1},
        colorbar='ul', colorbar_kw={'label': 'colorbar from lines'}

# Colorbars
ax = fig.subplot(122, title='On-the-fly colorbar')
data = state.rand(8, 8)
    data, cmap='Reds1', extend='both', colorbar='b',
    colorbar_kw={'length': 0.8, 'label': 'colorbar label'},
    data, color='gray7', lw=1.5,
    label='contour', legend='ul', legend_kw={'label': 'legend from contours'},
<matplotlib.contour.QuadContourSet at 0x7fec04c8e820>
import proplot as pplt
import numpy as np
N = 10
state = np.random.RandomState(51423)
fig, axs = pplt.subplots(
    nrows=2, share=False,
    refwidth='55mm', panelpad='1em',
    suptitle='Stacked colorbars demo'

# Repeat for both axes
args1 = (0, 0.5, 1, 1, 'grays', 0.5)
args2 = (0, 0, 0.5, 0.5, 'reds', 1)
args3 = (0.5, 0, 1, 0.5, 'blues', 2)
for j, ax in enumerate(axs):
    ax.format(xlabel='data', xlocator=np.linspace(0, 0.8, 5), title=f'Subplot #{j+1}')
    for i, (x0, y0, x1, y1, cmap, scale) in enumerate((args1, args2, args3)):
        if j == 1 and i == 0:
        data = state.rand(N, N) * scale
        x, y = np.linspace(x0, x1, N + 1), np.linspace(y0, y1, N + 1)
        m = ax.pcolormesh(x, y, data, cmap=cmap, levels=np.linspace(0, scale, 11))
        ax.colorbar(m, loc='l', label=f'dataset #{i + 1}')

Figure-wide colorbars and legends

In proplot, colorbars and legends can be added to the edge of figures using the figure methods proplot.figure.Figure.colorbar and proplot.figure.Figure.legend. These methods align colorbars and legends between the edges of the gridspec rather than the figure. As with axes colorbars and legends, if you draw multiple colorbars or legends on the same side, they are stacked on top of each other. To draw a colorbar or legend alongside particular row(s) or column(s) of the subplot grid, use the row, rows, col, or cols keyword arguments. You can pass an integer to draw the colorbar or legend beside a single row or column (e.g., fig.colorbar(m, row=1)), or pass a tuple to draw the colorbar or legend along a range of rows or columns (e.g., fig.colorbar(m, rows=(1, 2))). The space separation between the subplot grid edge and the colorbars or legends can be controlled with the space keyword, and the tight layout padding can be controlled with the pad keyword.

import proplot as pplt
import numpy as np
state = np.random.RandomState(51423)
fig, axs = pplt.subplots(ncols=3, nrows=3, refwidth=1.4)
for ax in axs:
    m = ax.pcolormesh(
        state.rand(20, 20), cmap='grays',
        levels=np.linspace(0, 1, 11), extend='both'
    suptitle='Figure colorbars and legends demo',
    abc='a.', abcloc='l', xlabel='xlabel', ylabel='ylabel'
fig.colorbar(m, label='column 1', ticks=0.5, loc='b', col=1)
fig.colorbar(m, label='columns 2 and 3', ticks=0.2, loc='b', cols=(2, 3))
fig.colorbar(m, label='stacked colorbar', ticks=0.1, loc='b', minorticks=0.05)
fig.colorbar(m, label='colorbar with length <1', ticks=0.1, loc='r', length=0.7)
<matplotlib.colorbar.Colorbar at 0x7fec040e2430>
import proplot as pplt
import numpy as np
state = np.random.RandomState(51423)
fig, axs = pplt.subplots(
    ncols=2, nrows=2, order='F', refwidth=1.7, wspace=2.5, share=False

# Plot data
data = (state.rand(50, 50) - 0.1).cumsum(axis=0)
for ax in axs[:2]:
    m = ax.contourf(data, cmap='grays', extend='both')
hs = []
colors = pplt.get_colors('grays', 5)
for abc, color in zip('ABCDEF', colors):
    data = state.rand(10)
    for ax in axs[2:]:
        h, = ax.plot(data, color=color, lw=3, label=f'line {abc}')

# Add colorbars and legends
fig.colorbar(m, length=0.8, label='colorbar label', loc='b', col=1, locator=5)
fig.colorbar(m, label='colorbar label', loc='l')
fig.legend(hs, ncols=2, center=True, frame=False, loc='b', col=2)
fig.legend(hs, ncols=1, label='legend label', frame=False, loc='r')
fig.format(abc='A', abcloc='ul', suptitle='Figure colorbars and legends demo')
for ax, title in zip(axs, ('2D {} #1', '2D {} #2', 'Line {} #1', 'Line {} #2')):
    ax.format(xlabel='xlabel', title=title.format('dataset'))

Added colorbar features

The proplot.axes.Axes.colorbar and proplot.figure.Figure.colorbar commands are somehwat more flexible than their matplotlib counterparts. The following core features are unique to proplot:

  • Calling colorbar with a list of Artists, a Colormap name or object, or a list of colors will build the required ScalarMappable on-the-fly. Lists of Artistss are used when you use the colorbar keyword with 1D commands like plot.

  • The associated colormap normalizer can be specified with the vmin, vmax, norm, and norm_kw keywords. The DiscreteNorm levels can be specified with values, or proplot will infer them from the Artist labels (non-numeric labels will be applied to the colorbar as tick labels). This can be useful for labeling discrete plot elements that bear some numeric relationship to each other.

Proplot also includes improvements for adding ticks and tick labels to colorbars. Similar to proplot.axes.CartesianAxes.format, you can flexibly specify major tick locations, minor tick locations, and major tick labels using the locator, minorlocator, formatter, ticks, minorticks, and ticklabels keywords. These arguments are passed through the Locator and Formatter constructor functions. Unlike matplotlib, the default ticks for discrete colormaps are restricted based on the axis length using DiscreteLocator. You can easily toggle minor ticks using tickminor=True.

Similar to axes panels, the geometry of proplot colorbars is specified with physical units (this helps avoid the common issue where colorbars appear “too skinny” or “too fat” and preserves their appearance when the figure size changes). You can specify the colorbar width locally using the width keyword or globally using the rc['colorbar.width'] setting (for outer colorbars) and the rc['colorbar.insetwidth'] setting (for inset colorbars). Similarly, you can specify the colorbar length locally with the length keyword or globally using the rc['colorbar.insetlength'] setting. The outer colorbar length is always relative to the subplot grid and always has a default of 1. You can also specify the size of the colorbar “extensions” in physical units rather than relative units using the extendsize keyword rather than matplotlib’s extendfrac. The default extendsize values are rc['colorbar.extend'] (for outer colorbars) and rc['colorbar.insetextend'] (for inset colorbars). See colorbar for details.

import proplot as pplt
import numpy as np
fig = pplt.figure(share=False, refwidth=2)

# Colorbars from lines
ax = fig.subplot(121)
state = np.random.RandomState(51423)
data = 1 + (state.rand(12, 10) - 0.45).cumsum(axis=0)
cycle = pplt.Cycle('algae')
hs = ax.line(
    data, lw=4, cycle=cycle, colorbar='lr',
    colorbar_kw={'length': '8em', 'label': 'line colorbar'}
    hs, loc='t', values=np.arange(0, 10),
    label='line colorbar', ticks=2

# Colorbars from a mappable
ax = fig.subplot(122)
m = ax.contourf(
    data.T, extend='both', cmap='algae',
    levels=pplt.arange(0, 3, 0.5)
    m, loc='r', length=1,  # length is relative
    label='interior ticks', tickloc='left'
    m, loc='ul', length=6,  # length is em widths
    label='inset colorbar', tickminor=True, alpha=0.5,
    suptitle='Colorbar formatting demo',
    xlabel='xlabel', ylabel='ylabel', titleabove=False

Added legend features

The proplot.axes.Axes.legend and proplot.figure.Figure.legend commands are somewhat more flexible than their matplotlib counterparts. The following core features are the same as matplotlib:

  • Calling legend without positional arguments will automatically fill the legend with the labeled artist in the the parent axes (when using proplot.axes.Axes.legend) or or the parent figure (when using proplot.figure.Figure.legend).

  • Legend labels can be assigned early by calling plotting comamnds with the label keyword (e.g., ax.plot(..., label='label')) or on-the-fly by passing two positional arguments to legend (where the first argument is the “handle” list and the second is the “label” list).

The following core features are unique to proplot:

  • Legend labels can be assigned for each column of a 2D array passed to a 1D plotting command using the labels keyword (e.g., labels=['label1', 'label2', ...]).

  • Legend labels can be assigned to ContourSets by passing the label keyword to a contouring command (e.g., contour or contourf).

  • A “handle” list can be passed to legend as the sole positional argument and the labels will be automatically inferred using get_label. Valid “handles” include Line2Ds returned by plot, BarContainers returned by bar, and PolyCollections returned by fill_between.

  • A composite handle can be created by grouping the “handle” list objects into tuples (see this matplotlib guide for more on tuple groups). The associated label will be automatically inferred from the objects in the group. If multiple distinct labels are found then the group is automatically expanded.

proplot.axes.Axes.legend and proplot.figure.Figure.legend include a few other useful features. To draw legends with centered rows, pass center=True or a list of lists of “handles” to legend (this stacks several single-row, horizontally centered legends and adds an encompassing frame behind them). To switch between row-major and column-major order for legend entries, use the order keyword (the default order='C' is row-major, unlike matplotlib’s column-major order='F'). To alphabetize the legend entries, pass alphabetize=True to legend. To modify the legend handles (e.g., plot or scatter handles) pass the relevant properties like color, linewidth, or markersize to legend (or use the handle_kw keyword). See proplot.axes.Axes.legend for details.

import proplot as pplt
import numpy as np
pplt.rc.cycle = '538'
fig, axs = pplt.subplots(ncols=2, span=False, share='labels', refwidth=2.3)
labels = ['a', 'bb', 'ccc', 'dddd', 'eeeee']
hs1, hs2 = [], []

# On-the-fly legends
state = np.random.RandomState(51423)
for i, label in enumerate(labels):
    data = (state.rand(20) - 0.45).cumsum(axis=0)
    h1 = axs[0].plot(
        data, lw=4, label=label, legend='ul',
        legend_kw={'order': 'F', 'title': 'column major'}
    h2 = axs[1].plot(
        data, lw=4, cycle='Set3', label=label, legend='r',
        legend_kw={'lw': 8, 'ncols': 1, 'frame': False, 'title': 'modified\n handles'}

# Outer legends
ax = axs[0]
ax.legend(hs1, loc='b', ncols=3, title='row major', order='C', facecolor='gray2')
ax = axs[1]
ax.legend(hs2, loc='b', ncols=3, center=True, title='centered rows')
axs.format(xlabel='xlabel', ylabel='ylabel', suptitle='Legend formatting demo')