[PierreRaybaut]

Closes #80. Part of DataLab-Platform/DataLab#453 (curve readability).

Problem

With a display scaling factor such as 125 % or 150 %, a curve of width 1.0 is drawn wider than one device pixel, and Qt's raster engine takes its slow path for wide pens. Noisy curves become several hundred times slower to draw.

Change

• CurveItem.drawCurve (and the error bars of ErrorBarCurveItem) draws thin pens with a cosmetic copy of the pen, i.e. one device pixel wide, through the new get_screen_pen() helper.
• This only applies on screen: raster paint engine without scaling transform. Printing and exporting (PDF/printer engines, or the scaled painter of QwtPlotRenderer) are not affected.
• The stored pen and the curve parameters (LineStyleParam.width) are unchanged.
• The new curve/cosmetic_hairline option of the plot configuration section (enabled by default) restores the previous behavior.

Measurements

Noisy curve, 2000 points, antialiased, CurveItem.drawSeries on a QImage:

Device pixel ratio	Option disabled	Option enabled
1.0	2.8 ms	2.5 ms
1.5	598 ms	3.6 ms

Tests

New plotpy/tests/unit/test_curve_screen_pen.py:

• thin and wide pens;
• other paint engines and scaled painters;
• configuration option;
• pixel-level rendering at device pixel ratio 2 (curves and error bars);
• stored parameters unchanged.

The plotpy/tests/unit and plotpy/tests/items suites pass locally, except test_edit_point_tool, which also fails locally without this change (mouse simulation).