How to use custom typeface for vertex labels in python igraph?

Question

In R igraph, there is an easy way to set a custom typeface for vertex labels and other texts on graph plots:

plot(G,vertex.label.family="DINPro")

However as I see, vertex_label_family parameter is not available in python igraph. It took some time while I have found some workaround to be able to set the label font, so I will post my solution here. And I'm wondering, if there are more elegant or easier solutions. First I tried to call cairo's context.select_font_face() on the igraph plot object's context, but sadly the draw() function in igraph.drawing.graph.DefaultGraphDrawer() overwrites it in each plot.redraw():

lo = graph.layout_fruchterman_reingold()
sf = cairo.PDFSurface("test.pdf",1280,1280)
bx = igraph.drawing.utils.BoundingBox(10, 10, 1260, 1260)
plot = igraph.plot(graph,layout=lo,target=sf,bbox=bx)
plot._ctx.select_font_face("DINPro", cairo.FONT_SLANT_NORMAL, cairo.FONT_WEIGHT_NORMAL)
plot.redraw()
plot.save()

So this doesn't work. I will post a simple solution what works.

deeenes deeenes · Accepted Answer · 2014-06-15T16:18:50

I could manage to set the typeface of vertex labels with replacing the DefaultGraphDrawer class with a new one, which only differs in two lines from the original. I simply copied the igraph/drawing/graph.py file from the site-packages directory of my python distribution, and I deleted every other classes except the DefaultDraphDrawer. After the imports, I added a line to import the AbstractCairoGraphDrawer, and modified the __all__[] list:

from igraph.drawing.graph import AbstractCairoGraphDrawer
__all__ = ["DefaultGraphDrawerFFsupport"]

Then I renamed the DefaultGraphDrawer class, modified the font selection part of it:

class DefaultGraphDrawerFFsupport(AbstractCairoGraphDrawer):
    ...
    def draw(...):
        ...
        fontFamily = graph.font if hasattr(graph, "font") else "sans-serif"
        context.select_font_face(fontFamily, cairo.FONT_SLANT_NORMAL, \
             cairo.FONT_WEIGHT_NORMAL)
        ...
    ...

I put this file (ig_drawing.py) in the directory of my module, and imported it. The I could set any typeface by passing the modified class as drawing_factory parameter for the plot() function:

from ig_drawing import *
lo = graph.layout_fruchterman_reingold()
sf = cairo.PDFSurface("test.pdf",1280,1280)
bx = igraph.drawing.utils.BoundingBox(10, 10, 1260, 1260)
df = igraph.drawing.graph.DefaultGraphDrawer(ctx, bx)
graph.font = "DINPro"
plot = graph.plot(graph,layout=lo,target=sf,bbox=bx,vertex_label=graph.vs["label"],drawer_factory=DefaultGraphDrawerFFsupport)
a.plot.save()

In case you don't set the graph.font, it uses "sans-serif". But also if you give a non existing font name, then without error message it falls back to your system's default sans font.

It looks like the default "sans-serif" typeface is hardcoded in igraph, both in plotting functions using cairo surfaces and svg. Any simpler way to change this?

How to use custom typeface for vertex labels in python igraph?

1 Answers