Improve graph rendering performance

[wawuwo]

Hi!

A quick and dirty attempt to deal with bad rendering performance of graphs.
It's not the best, but it should do the thing. I can see the actual difference when opening the sample circuit from #984 on my machine.

[SmallJoker]

Thank you for the PR. I tested the AppImage a little bit, and so far it does indeed fix the performance issue. No visual issues found.

[zergud]

good solution for lines!
remains drawArrowSymbols drawStarSymbols (!!) drawCircleSymbols

[iwbnwif]

I am not sure if it is related to this optimisation, but I have a line being drawn to infinity if I zoom in on the chart. It isn't there on the previous CI build.

Test1.sch.txt

[ra3xdh]

@wawuwo , Yes were considering a similar solution with points number decimation. The freezes are gone, but some problem still exist.

• The dashed curves look odd. See the screenshots from old (before Refactor drawing subsystem #723) and new version. This problem is not related to this fix and may be fixed by a separate PR.

old:

new:

• I confirm the problem reported by @iwbnwif Something strange happens when using Set diagram limits

@zergud The circles, arrows, and stars count usually should be equal to point number. The typical usecase is a Fourier simulation (arrows) and matching of the few points of experimental data (stars).

[ra3xdh]

The problems with dashed curve and Set limits are fixed. I will merge this once the CI finishes.

[ra3xdh]

Merged.

[zergud]

and yet, the behavior when using stars and circles is not correct when there are a large number of points in the dataset

black bold line - circle
red - stars

I think we can find a solution similar to the proposed algorithm for decimation of points
of course in another PR :)

[ra3xdh]

Octave package has the same behavior as Qucs-S when using stars line style. Here is the screenshot from Octave. Blue curve is stars 2000 points, redcurve is stars 20 points. So I would keep the stars/circles in Qucs-S as is.