feat: add ImPlotLegendFlags_Reverse (#640)

* Add ImPlotLegendFlags_Reverse

This is handy for making the order of legend items match the order of the data in stacked plots.
See #292

---------

Co-authored-by: Breno Cunha Queiroz <brenocq.br@gmail.com>

Author: howprice

implot.cpp

@@ -652,7 +652,7 @@ bool ShowLegendEntries(ImPlotItemGroup& items, const ImRect& legend_bb, bool hov
     }
     // render
     for (int i = 0; i < num_items; ++i) {
-        const int idx           = indices[i];
+        const int idx           = ImHasFlag(items.Legend.Flags, ImPlotLegendFlags_Reverse) ? indices[num_items - 1 - i] : indices[i];
         ImPlotItem* item        = items.GetLegendItem(idx);
         const char* label       = items.GetLegendLabel(idx);
         const float label_width = ImGui::CalcTextSize(label, nullptr, true).x;

implot.h

@@ -195,6 +195,7 @@ enum ImPlotLegendFlags_ {
     ImPlotLegendFlags_Outside         = 1 << 4, // legend will be rendered outside of the plot area
     ImPlotLegendFlags_Horizontal      = 1 << 5, // legend entries will be displayed horizontally
     ImPlotLegendFlags_Sort            = 1 << 6, // legend entries will be displayed in alphabetical order
+    ImPlotLegendFlags_Reverse         = 1 << 7, // legend entries will be displayed in reverse order
 };
 
 // Options for mouse hover text (see SetupMouseText)

implot_demo.cpp

@@ -379,6 +379,7 @@ void Demo_ShadedPlots() {
     ImGui::DragFloat("Alpha",&alpha,0.01f,0,1);
 
     if (ImPlot::BeginPlot("Shaded Plots")) {
+        ImPlot::SetupLegend(ImPlotLocation_NorthWest, ImPlotLegendFlags_Reverse); // reverse legend to match vertical order on plot
         ImPlot::PushStyleVar(ImPlotStyleVar_FillAlpha, alpha);
         ImPlot::PlotShaded("Uncertain Data",xs,ys1,ys2,1001);
         ImPlot::PlotLine("Uncertain Data", xs, ys, 1001);