matplotlib: add scale bars to axes

scalebars.py

# -*- coding: utf-8 -*-
# -*- mode: python -*-
# Adapted from mpl_toolkits.axes_grid1
# LICENSE: Python Software Foundation (http://docs.python.org/license.html)

from matplotlib.offsetbox import AnchoredOffsetbox
class AnchoredScaleBar(AnchoredOffsetbox):
    def __init__(self, transform, sizex=0, sizey=0, labelx=None, labely=None, loc=4,
                 pad=0.1, borderpad=0.1, sep=2, prop=None, barcolor="black", barwidth=None, 
                 **kwargs):
        """
        Draw a horizontal and/or vertical  bar with the size in data coordinate
        of the give axes. A label will be drawn underneath (center-aligned).

        - transform : the coordinate frame (typically axes.transData)
        - sizex,sizey : width of x,y bar, in data units. 0 to omit
        - labelx,labely : labels for x,y bars; None to omit
        - loc : position in containing axes
        - pad, borderpad : padding, in fraction of the legend font size (or prop)
        - sep : separation between labels and bars in points.
        - **kwargs : additional arguments passed to base class constructor
        """
        from matplotlib.patches import Rectangle
        from matplotlib.offsetbox import AuxTransformBox, VPacker, HPacker, TextArea, DrawingArea
        bars = AuxTransformBox(transform)
        if sizex:
            bars.add_artist(Rectangle((0,0), sizex, 0, ec=barcolor, lw=barwidth, fc="none"))
        if sizey:
            bars.add_artist(Rectangle((0,0), 0, sizey, ec=barcolor, lw=barwidth, fc="none"))

        if sizex and labelx:
            self.xlabel = TextArea(labelx)
            bars = VPacker(children=[bars, self.xlabel], align="center", pad=0, sep=sep)
        if sizey and labely:
            self.ylabel = TextArea(labely)
            bars = HPacker(children=[self.ylabel, bars], align="center", pad=0, sep=sep)

        AnchoredOffsetbox.__init__(self, loc, pad=pad, borderpad=borderpad,
                                   child=bars, prop=prop, frameon=False, **kwargs)

        
def add_scalebar(ax, matchx=True, matchy=True, hidex=True, hidey=True, **kwargs):
    """ Add scalebars to axes

    Adds a set of scale bars to *ax*, matching the size to the ticks of the plot
    and optionally hiding the x and y axes

    - ax : the axis to attach ticks to
    - matchx,matchy : if True, set size of scale bars to spacing between ticks
                    if False, size should be set using sizex and sizey params
    - hidex,hidey : if True, hide x-axis and y-axis of parent
    - **kwargs : additional arguments passed to AnchoredScaleBars

    Returns created scalebar object
    """
    def f(axis):
        l = axis.get_majorticklocs()
        return len(l)>1 and (l[1] - l[0])
    
    if matchx:
        kwargs['sizex'] = f(ax.xaxis)
        kwargs['labelx'] = str(kwargs['sizex'])
    if matchy:
        kwargs['sizey'] = f(ax.yaxis)
        kwargs['labely'] = str(kwargs['sizey'])
        
    sb = AnchoredScaleBar(ax.transData, **kwargs)
    ax.add_artist(sb)

    if hidex : ax.xaxis.set_visible(False)
    if hidey : ax.yaxis.set_visible(False)
    if hidex and hidey: ax.set_frame_on(False)

    return sb

Thanks for this, I've used it for a few months now. It looks like it is adapted from AnchoredSizeBar ( in mpl_toolkits.axes_grid1.anchored_artists). That one can only make a horizontal bar, this one can make both horizontal and vertical bars.

Is axes_grid2 a real thing, or is that a typo of axes_grid1?

Glad it's been of use! axes_grid2 is a typo; thanks for catching it.

The last revision should fix issues with the bars not displaying in recent versions of matplotlib. Plus now you can customize color and width

Hi @dmeliza thank you for sharing this -- it's helpful!

Not sure if you're updating this but thought I'd let you know that minimumdescent (like in line 32) is now deprecated:
https://matplotlib.org/stable/api/api_changes.html#minimumdescent-parameter-property-of-textarea

Looks like it won't affect this since you set it to False anyway, but removing the parameter avoids a warning.

Thanks! Have not been using this myself any more, but appreciate the fix should I need to dust off my matplotlib skillz. Removed the offending parameter.

Excellent, thanks!

Amazing! Thanks, @dmeliza!

AWESOME! Thanks a lot!