Plot: Legend improvements (#410)

* initial work on markers

* clippy fix

* simplify marker

* use option for color

* prepare for more demo plots

* more improvements for markers

* some small adjustments

* better highlighting

* don't draw transparent lines

* use transparent color instead of option

* don't brighten curves when highlighting

* Initial changes to lengend:
* Font options
* Position options
* Internal cleanup

* draw legend on top of curves

* update changelog

* fix legend checkboxes

* simplify legend

* remove unnecessary derives

* remove config from legend entries

* avoid allocations and use line_segment

* compare against transparent color

* create new Points primitive

* fix doctest

* some cleanup and fix hover

* common interface for lines and points

* clippy fixes

* reduce visibilities

* update legend

* clippy fix

* change instances of "curve" to "item"

* change visibility

* Update egui/src/widgets/plot/mod.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui/src/widgets/plot/mod.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui_demo_lib/src/apps/demo/plot_demo.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui_demo_lib/src/apps/demo/plot_demo.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* changes based on review

* add legend to demo

* fix test

* move highlighted items to front

* dynamic plot size

* add legend again

* remove height

* clippy fix

* update changelog

* minor changes

* Update egui/src/widgets/plot/legend.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui/src/widgets/plot/legend.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* Update egui/src/widgets/plot/legend.rs

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

* changes based on review

* add functions to mutate legend config

* use horizontal_align

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

CHANGELOG.md

@@ -8,6 +8,7 @@ NOTE: [`eframe`](eframe/CHANGELOG.md), [`egui_web`](egui_web/CHANGELOG.md) and [
 ## Unreleased
 
 ### Added ⭐
+* [Plot legend improvements](https://github.com/emilk/egui/pull/410).
 * [Line markers for plots](https://github.com/emilk/egui/pull/363).
 * Add right and bottom panels (`SidePanel::right` and `Panel::bottom`).
 * Add resizable panels.