PGF backend: Fix vertical positioning of multi-line text

[adamreeve]

This addresses issue #3000. The horizontal aligment is maintained, but for multi-line text the vertical alignment is applied manually in a similar way to the SVG backend.

[adamreeve]

Thanks, I don't think the label appearing outside the figure is specific to the pgf backend with this patch though, I get the same thing with other backends. You can use figure.tight_layout() to make sure everything fits in nicely.

[tacaswell]

@pwuertz Can you review this?

[tacaswell]

@adamreeve Can you add an entry to CHANGELOG?

[adamreeve]

Hi @tacaswell, I added the CHANGELOG entry and rebased to avoid a merge.

[pwuertz]

Code looks very good to me, although I think it is the function calling the backend's draw_text() method that needs to be fixed. The problem originates from the fact that the mtext instances given to the backend are completely wrong. At some point matplotlib splits up lines, calculates the vertical positions and calls draw_text multiple times, where it should provide individual, correct mtext instances for all lines.

[adamreeve]

Yeah, I had a quick look and Text._get_layout in lib/matplotlib/text.py is where the text is split into multiple lines. These lines are then used by Text.draw which calls the renderer for each line and passes the Text object in as the mtext parameter.

An alternative approach could be to let the renderer say that it can handle multi-line text, then let it split the lines up itself and position them using the mtext info or just draw all the text with something like a parbox as you mentioned in #3000.

[pwuertz]

Here is the quick fix that omits mtext in cases where it isn't valid (#3017).

@adamreeve Thats right, it is probably much cleaner to move the _get_layout code to the base backend since it depends on the renderer anyway and allow backend subclasses to handle multiple lines for themselves if necessary, like using parbox for latex. Unfortunately I don't think it is possible to do that reliably in latex for the reasons I mentioned in #3000.

[adamreeve]

Ok thanks, I'll close this PR then. Right, that makes sense, so providing correct mtext instances for the separate lines would be the most useful for the PGF backend.

[pwuertz]

Thanks for your work and thoughts on this.