[cairo] Line widths and coordinate system transformations

[Holger Berndt]

For my gtkmm-based application, I am writing a custom xy-plot widget
into which I can draw some lines and curves using cairomm. To get rid of
permanent manual coordinate transformations, it is very handy to
translate and scale the drawing coordinate system into alignment with
the plot axis. This works very well.

I have, however, a problem with the resulting line width. Usually, x
and y axis are not equally scaled which results in the pen having
different widths depending on the direction of the line.

I have attached a sample screenshot to illustrate the problem (I hope
that's okay with the list -- it's really tiny). In this example, the
horizontal axis goes from 0 to 9, whereas the vertical axis goes from 0
to 1. Consequently, lines that are drawn horizontally are much thicker
than vertical lines (the requested line width with
Cairo::Context::set_line_width() remained unchanged).

What I would like to have constant user-visible line widths in all
directions.

As a bonus, it would also be nice to specify a constant line width in
pixels, completely independant of the scaling. But that'd really be a
bonus -- for now, I'd already be happy with scale-dependant but
direction-independant line widths.

I am aware that I could achieve that by using the same scale in x and y
direction. However, that would make drawing much harder, as I would
have to do the transformations for each point individually and
manually, which is something that I'd really, really like to avoid.

In particular, this very same effect seems to imply that it is not
possible to draw an ellipse outline with constant line width with
Cairo. Is that correct?

Thanks in advance for any comments,
Holger

PS: This mail was originally sent to the gtkmm mailing list, but the
lack of feedback made me believe that it's actually a cairo question,
not a gtk question.
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