How to add a fixed-width border to subplot

Question

I want to add a coloured border to some subplots with a fixed width specified in pixels. I wrote the following function to do so by adding a Rectangle patch to the figure behind the axes.

def add_subplot_border(ax, width=0, color=None):
    fig = ax.get_figure()

    # Convert bottom-left and top-right to display coordinates
    x0, y0 = ax.transAxes.transform((0, 0))
    x1, y1 = ax.transAxes.transform((1, 1))

    # Adjust margins
    x0 -= width
    x1 += width
    y0 -= width
    y1 += width

    # Convert back to Axes coordinates
    x0, y0 = ax.transAxes.inverted().transform((x0, y0))
    x1, y1 = ax.transAxes.inverted().transform((x1, y1))

    rect = plt.Rectangle((x0, y0), x1-x0, y1-y0,
                         color=color,
                         transform=ax.transAxes,
                         zorder=-1)

    fig.patches.append(rect)

This appears to be a good starting point, but when the figure is resized the relative thickness of the border changes too. How can I specify a transform to scale and translate the patch to appear as a fixed-width border regardless of window scaling? Or, is there a better way to approach this?

Original figure

Scaled figure - uneven border

How about colouring the axes themselves instead of underlaying a coloured Rect? See this answer for how to do it. See also this answer on how to set the linewidth of the axis. — Thomas Kühn
– Thomas Kühn, Commented Aug 1, 2017 at 20:02
@ThomasKühn that might be a good solution to my immediate problem, but I was hoping to extend the idea to colour the entire subplot background — user3419537
– user3419537, Commented Aug 2, 2017 at 11:35
See my answer for the fixed-width Rectangle. If you want to colour the entire subplot-background, you can use ax.set_facecolor(). See for instance this answer — Thomas Kühn
– Thomas Kühn, Commented Aug 2, 2017 at 12:44
Sorry, I mean the background of the subplot where the ticks and labels reside, not the Axes itself. — user3419537
– user3419537, Commented Aug 2, 2017 at 12:53

Thomas Kühn · Accepted Answer · 2017-08-02 12:45:47Z

3

Instead of calculating a margin and drawing a Rectangle with that extra width (which then get's overlayed by the Axis, you can give the Rectangle a line width (in points) that is preserved upon re-scaling. Note though, that the line is always centred on the border of the Rectangle, so if you want, say, a 5 point frame around your axis, you should request a line width of 10 (or possibly 11).

I adjusted your function slightly and added a use case example:

from matplotlib import pyplot as plt

def add_subplot_border(ax, width=1, color=None ):

    fig = ax.get_figure()

    # Convert bottom-left and top-right to display coordinates
    x0, y0 = ax.transAxes.transform((0, 0))
    x1, y1 = ax.transAxes.transform((1, 1))

    # Convert back to Axes coordinates
    x0, y0 = ax.transAxes.inverted().transform((x0, y0))
    x1, y1 = ax.transAxes.inverted().transform((x1, y1))

    rect = plt.Rectangle(
        (x0, y0), x1-x0, y1-y0,
        color=color,
        transform=ax.transAxes,
        zorder=-1,
        lw=2*width+1,
        fill=None,
    )
    fig.patches.append(rect)


if __name__ == '__main__':
    fig,axes = plt.subplots(ncols=2,nrows=2,figsize=(8,8))

    colors = 'brgy'
    widths = [1,2,4,8]

    for ax,col,w in zip(axes.reshape(-1),colors, widths):
        add_subplot_border(ax,w,col)

    plt.show()

This is the original figure:

and this is the scaled figure (the lines look thinner because I increased the figure size):

edited Aug 2, 2017 at 12:45

answered Aug 2, 2017 at 12:30

Thomas Kühn

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7 Comments

user3419537 Over a year ago

Not quite perfect (may be due to my use of figure.autolayout), but a good improvement on what I had. Thanks

Thomas Kühn Over a year ago

@user3419537 If you tell me what needs improvement, I can try to work on it.

user3419537 Over a year ago

The width of the line remains a fixed size now, but the bounds of the rectangle are shifting relative to the axes area upon resizing

Thomas Kühn Over a year ago

@user3419537 that's weird -- doesn't happen for me. Can you give me your exact autolayout command?

user3419537 Over a year ago

Ahhh I'm an idiot. I left the margin adjustments from the original function in place. Now it works nicely!

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