Add BeginPopupContextLegend and EndPopup

EndPopup is just a wrapper around ImGui::EndPopup.

BeginPopupContextLegend provides easy-to-use context menus for legend
entries along the lines of ImGui::BeginPopupContextItem.

implot.cpp

@@ -1892,6 +1892,25 @@ void EndLegendDragDropSource() {
     ImGui::EndDragDropSource();
 }
 
+bool BeginPopupContextLegend(const char* label_id, ImGuiMouseButton mouse_button) {
+    ImPlotContext& gp = *GImPlot;
+    IM_ASSERT_USER_ERROR(gp.CurrentPlot != NULL, "BeginLegendPopup() needs to be called between BeginPlot() and EndPlot()!");
+    ImGuiWindow* window = GImGui->CurrentWindow;
+    if (window->SkipItems)
+        return false;
+    ImGuiID id = ImGui::GetID(label_id);
+    if (ImGui::IsMouseReleased(mouse_button)) {
+        ImPlotItem* item = gp.CurrentPlot->Items.GetByKey(id);
+        if (item && item->Highlight)
+            ImGui::OpenPopupEx(id);
+    }
+    return ImGui::BeginPopupEx(id, ImGuiWindowFlags_AlwaysAutoResize | ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoSavedSettings);
+}
+
+void EndPopup() {
+    ImGui::EndPopup();
+}
+
 //-----------------------------------------------------------------------------
 // STYLING
 //-----------------------------------------------------------------------------

implot.h

@@ -522,6 +522,11 @@ bool BeginLegendDragDropSource(const char* label_id, ImGuiDragDropFlags flags =
 // End legend drag and drop source.
 void EndLegendDragDropSource();
 
+// Begin a popup for a legend entry.
+bool BeginPopupContextLegend(const char* label_id, ImGuiMouseButton mouse_button = 1);
+// End a popup for a legend entry.
+void EndPopup();
+
 //-----------------------------------------------------------------------------
 // Miscellaneous
 //-----------------------------------------------------------------------------

implot_demo.cpp

@@ -237,10 +237,19 @@ void ShowDemoWindow(bool* p_open) {
             ys2[i] = xs2[i] * xs2[i];
         }
         ImGui::BulletText("Anti-aliasing can be enabled from the plot's context menu (see Help).");
+        ImGui::BulletText("Right click on a legend item to bring up its context menu");
         if (ImPlot::BeginPlot("Line Plot", "x", "f(x)")) {
             ImPlot::PlotLine("sin(x)", xs1, ys1, 1001);
             ImPlot::SetNextMarkerStyle(ImPlotMarker_Circle);
             ImPlot::PlotLine("x^2", xs2, ys2, 11);
+            if (ImPlot::BeginPopupContextLegend("sin(x)")) {
+                ImGui::Text("Context menu for sin(x)");
+                ImPlot::EndPopup();
+            }
+            if (ImPlot::BeginPopupContextLegend("x^2")) {
+                ImGui::Text("Context menu for x^2");
+                ImPlot::EndPopup();
+            }
             ImPlot::EndPlot();
         }
     }