Plot: 2nd and 3rd axis support

[gucio321]

          @gucio321 

"2nd and 3rd axis":

old code was:

giu.Plot("Rotation").Flags(giu.PlotFlagsYAxis2).Plots( .... )

now it doesn't work because PlotFlagsYAxis2 doesn't exist, in flags.go it is commented out:

const (
	PlotFlagsNone        = PlotFlags(imgui.PlotFlagsNone)
	PlotFlagsNoTitle     = PlotFlags(imgui.PlotFlagsNoTitle)
	PlotFlagsNoLegend    = PlotFlags(imgui.PlotFlagsNoLegend)
	PlotFlagsNoMenus     = PlotFlags(imgui.PlotFlagsNoMenus)
	PlotFlagsNoBoxSelect = PlotFlags(imgui.PlotFlagsNoBoxSelect)
	// 	PlotFlagsNoMousePos  = PlotFlags(imgui.PlotFlagsNoMousePos)
	// 	PlotFlagsNoHighlight = PlotFlags(imgui.PlotFlagsNoHighlight)
	// PlotFlagsNoChild = PlotFlags(imgui.PlotFlagsNoChild).
	PlotFlagsEqual = PlotFlags(imgui.PlotFlagsEqual)
	// 	PlotFlagsYAxis2      = PlotFlags(imgui.PlotFlagsYAxis2)
	// 	PlotFlagsYAxis3      = PlotFlags(imgui.PlotFlagsYAxis3)
	// 	PlotFlagsQuery       = PlotFlags(imgui.PlotFlagsQuery)
	PlotFlagsCrosshairs = PlotFlags(imgui.PlotFlagsCrosshairs)
	// 	PlotFlagsAntiAliased = PlotFlags(imgui.PlotFlagsAntiAliased)
	PlotFlagsCanvasOnly = PlotFlags(imgui.PlotFlagsCanvasOnly)
)

I have no idea how to activate second/third axis

Originally posted by @francmarx in #632 (comment)

[runrc]

Is this likely to be addressed anytime soon? It would be really useful to plot using multiple axis.

[gucio321]

ok, let me take a look

[gucio321]

cimgui-go ref:

// original name: ImAxis_
type PlotAxisEnum int32

const (
        AxisX1    PlotAxisEnum = 0
        AxisX2    PlotAxisEnum = 1
        AxisX3    PlotAxisEnum = 2
        AxisY1    PlotAxisEnum = 3
        AxisY2    PlotAxisEnum = 4
        AxisY3    PlotAxisEnum = 5
        AxisCOUNT PlotAxisEnum = 6
)

(cimplot_enums.go)

func PlotSetAxes(x_axis PlotAxisEnum, y_axis PlotAxisEnum) {
        C.ImPlot_SetAxes(C.ImAxis(x_axis), C.ImAxis(y_axis))
}

(cimplot_funcs.go)

// Select which axis/axes will be used for subsequent plot elements.
IMPLOT_API void SetAxis(ImAxis axis);
IMPLOT_API void SetAxes(ImAxis x_axis, ImAxis y_axis);

[gucio321]

ok @runrc I opened #805

As I said, I don't have much free time at the moment as I'm in the middle of my examination session 😄, but I added some stuff that involves cimgui-go experience. What needs to be done now is improving giu api.
Feel free to finish my work and open PR ;-)