Re: [Matplotlib-users] x-axis Tick Label Alignment w/Cairo

[Michael D. <md...@st...>]

Thanks for this.  It's been a long-standing bug that text is handled by 
bounding box, but it's been difficult to find a way forward without 
breaking backward compatibility.

I've filed an issue for this here.

https://github.com/matplotlib/matplotlib/issues/1121

It may be, in the long run, that we want to fix the other backends to 
use baselines rather than fixing Cairo to use bounding boxes.

Mike

On 08/20/2012 01:07 PM, Freddie Witherden wrote:
> On 19/08/12 23:48, Freddie Witherden wrote:
>> Hello,
>>
>> Using the Cairo backend with the following snippet:
>>
>> from matplotlib.figure import Figure
>> from matplotlib.artist import setp
>> from matplotlib.backends.backend_agg import FigureCanvasAgg
>> from matplotlib.backends.backend_cairo import FigureCanvasCairo
>> import numpy as np
>>
>> fig = Figure()
>> ax1 = fig.add_axes([0.1, 0.1, 0.8, 0.8])
>> x = np.arange(0, 10, 0.2)
>>
>> ax1.plot(x, np.sin(x))
>> ax1.xaxis.set_ticklabels(['Apr', 'Jul', 'Oct', '\'12', 'Apr', 'Jul'])
>> setp(ax1.yaxis.get_ticklabels(), visible=False)
>>
>> FigureCanvasCairo(fig).print_figure('test.png')
>>
>>
>> Results in the x-axis tick labels being significantly displaced.
>> Specifically, "Oct" and "'12" are positioned far closer to the axis than
>> either "Apr" or "Jul".
>>
>> With the AGG backend all of the labels are roughly aligned to the
>> baseline of the font -- give or take a pixel.
>>
>> I have observed this on a Gentoo and Debian system, both running 1.1.1,
>> albeit with different default fonts.
>>
>> Although I am not completely sure it appears as if a label contains a
>> glyph that extends below the baseline, e.g. 'p' or 'J', that the label
>> is forced away from the axis.
>>
>> Can anyone suggest a workaround for this (or explain where I am going
>> wrong)?
> I have been looking into this issue today and it /appears/ to be because
> the Cairo backend -- or more precisely -- cairo::show_text method takes
> a y-coordinate relative to the baseline.  This makes sense given that
> the descent of a font is a well-defined concept in Cairo (and can be
> extracted via the font_extents method).  However, as far as I can tell,
> matplotlib expects the draw_text method to provide an absolute y-coordinate.
>
> The result is that any text with a descender is pushed downwards
> (assuming a default rotation angle).  This can be visualized by
>
> setp(ax1.xaxis.get_ticklabels(), backgroundcolor='r')
>
> The solution is to have the draw_text method account for any descenders.
>   After a bit of juggling the relevant portion of draw_text becomes:
>
>             ctx = gc.ctx
>             ctx.new_path()
>             #ctx.move_to (x, y)
>             ctx.select_font_face (prop.get_name(),
>                                   self.fontangles [prop.get_style()],
>                                   self.fontweights[prop.get_weight()])
>
>             size = prop.get_size_in_points() * self.dpi / 72.0
>             ctx.set_font_size(size)
>
>             y_bearing, w, h = ctx.text_extents(s.encode("utf-8"))[1:4]
>             ctx.move_to(x, y - (h + y_bearing))
>
>             ctx.save()
>             if angle:
>                ctx.rotate (-angle * np.pi / 180)
>             #ctx.set_font_size (size)
>             ctx.show_text (s.encode("utf-8"))
>             ctx.restore()
>
> where commented lines highlight modifications (specifically, the
> movement of the set_font_size and move_to calls).  In my limited testing
> this fixes the aforementioned issues.  Discrepancies between baselines
> are now limited to +/- 1px (the same as with the AGG backend).
> Eliminating these one pixel misalignments is rather difficult so long as
> text rendering is performed relative to the bounding box as opposed to a
> baseline.
>
> Regards, Freddie.
>
>
>
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