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)