python-igraph

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Describe the bug The layout coordination systems in matplotlib and cairo are using inverted y-axis from each other.

To reproduce

g = ig.Graph.Famous('Zachary')
ig.plot(g)

yields

while

fig, ax = plt.subplots()
ig.plot(g, target=ax)

yields

It would be nice if the two coordination systems would align. Of course, I understand that both systems have their own drawing systems and Cairo probably uses a top-left coordinate system (i.e. positive y-axis goes down in the picture) while matplotlib uses a figure coordinate system (i.e. positive y-axis goes up in the picture). To me, it seems most natural to stick to a matplotlib type coordinate system. Could we perhaps simply always invert the y coordinate before drawing in Cairo?

Version information Development version from develop branch.

ax.invert_yaxis() before or after the plot inverts the y axis in mpl... would you like me to invert it by default?

I think we should stick to the defaults with Matplotlib and invert the Cairo plot instead. I'd vote for that because it is not nice if igraph messes around with the axis direction of the ax object that the user passes to the plot function (unless we are careful and revert it when we are done, even if we exit with an exception).

Cairo is also the lower level drawing interface, so there it makes sense that you setup your own coordinate system for drawing. Perhaps it even has direct support for custom coordinate systems?

In images, the y coordinate is usually top to bottom, I guess that's that @ntamas has done there

On Wed, Dec 15, 2021, at 16:51, Vincent Traag wrote:

Cairo is also the lower level drawing interface, so there it makes sense that you setup your own coordinate system for drawing. Perhaps it even has direct support for custom coordinate systems?

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Cairo has support for arbitrary transformation matrices so you can "push" a transformation to a Cairo context; this allows you to flip the axes, or scale them, or rotate/skew the whole thing. However, if we flip the Y axis permanently, we break the code for people who have drawn additional stuff on a Cairo context manually, plus we may potentially break the code that aligns labels above/below the vertices. I'd probably mess around with the call to layout.fit_into(...) in the graph drawing code; this is the part that takes the layout and scales the coordinates in a way that they fit into the bounding box that the user wants to draw into. Here we could flip the Y axis in the layout before mapping it to Cairo-space.

invert_y=None -> Cairo only?

Something like that, yes.

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