Transform from data to figure coordinates

Question

Similar to this post, I would like to transform my data coordinates to figure coordinates. Unfortunately, the transformation tutorial doesn't seem to talk about it. So I came up with something analogous to the answer by wilywampa, but for some reason, there is something wrong and I can't figure it out:

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.patches import ConnectionPatch

t = [
    0, 6.297, 39.988, 46.288, 79.989, 86.298, 120.005, 126.314, 159.994,
    166.295, 200.012, 206.314, 240.005, 246.301, 280.05, 286.35, 320.032,
    326.336, 360.045, 366.345, 480.971, 493.146, 1080.117, 1093.154, 1681.019,
    1692.266, 2281.008, 2293.146, 2881.014, 2893.178, 3480.988, 3493.149,
    4080.077, 4092.298, 4681.007, 4693.275, 5281.003, 5293.183, 5881.023,
    5893.188, 6481.002, 6492.31
]
y = np.zeros(len(t))


fig, (axA, axB) = plt.subplots(2, 1)
fig.tight_layout()
for ax in (axA, axB):
    ax.set_frame_on(False)
    ax.axes.get_yaxis().set_visible(False)

axA.plot(t[:22], y[:22], c='black')
axA.plot(t[:22], y[:22], 'o', c='#ff4500')
axA.set_ylim((-0.05, 1))
axB.plot(t, y, c='black')
axB.plot(t, y, 'o', c='#ff4500')
axB.set_ylim((-0.05, 1))
pos1 = axB.get_position()
pos2 = [pos1.x0, pos1.y0 + 0.3, pos1.width, pos1.height]
axB.set_position(pos2)

trans = [
    # (ax.transAxes + ax.transData.inverted()).inverted().transform for ax in 
    (fig.transFigure + ax.transData.inverted()).inverted().transform for ax in
    (axA, axB)
]

con1 = ConnectionPatch(
    xyA=trans[0]((0, 0)), xyB=(0, 0.1), coordsA="figure fraction",
    coordsB="data", axesA=axA, axesB=axB, color="black"
)
con2 = ConnectionPatch(
    xyA=(500, 0), xyB=(500, 0.1), coordsA="data", coordsB="data",
    axesA=axA, axesB=axB, color="black"
)
print(trans[0]((0, 0)))
axB.add_artist(con1)
axB.add_artist(con2)
plt.show()

The line on the left is supposed to go to (0, 0) of the upper axis, but it doesn't. The same happens btw if I try to convert to axes coordinates, so there seems be to something fundamentally wrong.

The reason why I want to use figure coords is because I don't actually want the line to end at (0, 0), but slightly below the '0' tick label. I cannot do that in data coords so I tried to swap to figure coods.

JohanC · Accepted Answer · 2021-02-05T11:34:40.657

Adapting the second example from this tutorial code, it seems no special combinations of transforms is needed. You can use coordsA=axA.get_xaxis_transform(), if x is in data coordinates and y in figure coordinates. Or coordsA=axA.transData if x and y are both in data coordinates. Note that when using data coordinates you are allowed to give coordinates outside the view window; by default a ConnectionPatch isn't clipped.

The following code uses z-order to put the connection lines behind the rest and adds a semi-transparent background to the tick labels of axA (avoiding that the text gets crossed out by the connection line):

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.patches import ConnectionPatch

t = [0, 6.297, 39.988, 46.288, 79.989, 86.298, 120.005, 126.314, 159.994, 166.295, 200.012, 206.314, 240.005, 246.301, 280.05, 286.35, 320.032, 326.336, 360.045, 366.345, 480.971, 493.146, 1080.117, 1093.154, 1681.019, 1692.266, 2281.008, 2293.146, 2881.014, 2893.178, 3480.988, 3493.149, 4080.077, 4092.298, 4681.007, 4693.275, 5281.003, 5293.183, 5881.023, 5893.188, 6481.002, 6492.31]
y = np.zeros(len(t))

fig, (axA, axB) = plt.subplots(2, 1)
fig.tight_layout()

for ax in (axA, axB):
    ax.set_frame_on(False)
    ax.axes.get_yaxis().set_visible(False)

axA.plot(t[:22], y[:22], c='black')
axA.plot(t[:22], y[:22], 'o', c='#ff4500')
axA.set_ylim((-0.05, 1))
axB.plot(t, y, c='black')
axB.plot(t, y, 'o', c='#ff4500')
axB.set_ylim((-0.05, 1))
pos1 = axB.get_position()
pos2 = [pos1.x0, pos1.y0 + 0.3, pos1.width, pos1.height]
axB.set_position(pos2)

con1 = ConnectionPatch(xyA=(0, 0.02), coordsA=axA.get_xaxis_transform(),
                       xyB=(0, 0.05), coordsB=axB.get_xaxis_transform(),
                       # linestyle='--', color='black', zorder=-1)
                       linestyle='--', color='darkgrey', zorder=-1)
con2 = ConnectionPatch(xyA=(500, 0.02), coordsA=axA.get_xaxis_transform(),
                       xyB=(500, 0.05), coordsB=axB.get_xaxis_transform(),
                       linestyle='--', color='darkgrey', zorder=-1)
fig.add_artist(con1)
fig.add_artist(con2)

for lbl in axA.get_xticklabels():
    lbl.set_backgroundcolor((1, 1, 1, 0.8))
plt.show()

Hey, thanks! Sorry, I just realized that I didn't make this clear. The reason why I want to use figure coords is because I don't actually want the line to end at `(0, 0)`, but slightly below the '0' tick label. I cannot do that in data coords so I tried to swap to figure coods. — mapf, Feb 05 '21 at 10:52
You could use `axA.get_xaxis_transform()` instead of `axA.transData` to have the x in data coordinates and the y in axes coordinates. — JohanC, Feb 05 '21 at 10:58
Great, thank you, this works. For completeness sake, how would you go about transforming into figure coords anyways (instead of axes coods)? — mapf, Feb 05 '21 at 12:19
But isn't that what I tried? I just really wonder where the offset is coming from and why there is only one in x but not in y. — mapf, Feb 06 '21 at 07:34
The problem in your code is that `ax.transData` doesn't yet have the correct transformation. The call to `tight_layout()` moved the positions of the axes but didn't update the internal transformations. You can force a draw, calling `fig.canvas.draw()` just before you want to use `ax.transData`. — JohanC, Feb 07 '21 at 01:17
Calling `fig.canvas.draw()` before defining the transformations did the trick! But `tight_layout()` wasn't responsible alone, since removing it, didn't have an effect for me. Anyways, I also found another curious thing. If I execute the same code on another machine, the plot looks completely different (it's all moved to the bottom and partly cutoff). I updated matplotlib on that machine but that didn't help. Oh well. — mapf, Feb 08 '21 at 11:01
I learned about ConnectionPatch after trying to chain some transforms manually and use Line2D. This is so much better, thanks for writing this answer! — LucasB, Nov 03 '22 at 10:19

score 1 · Answer 2 · answered Sep 13 '22 at 09:52

1

Possible answer to your last comment: As you're dealing with figure coords, these can change depending on your screen resolution. So if your other machine has a different res then this could be why its changing. You'll have to look into using Axes coords instead if you don't want these random changes.

answered Sep 13 '22 at 09:52

GeoDigg

11
1

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Transform from data to figure coordinates

2 Answers2