PlotHistogram: Fixed zero-line position when manually specifying min<0 and max>0. (#4349)

+ amended demo tweaks by ocornut

docs/CHANGELOG.txt

@@ -47,6 +47,7 @@ Other Changes:
   the whole widget is clipped.
 - TextUnformatted: Accept null ranges including (NULL,NULL) without asserting, in order to conform
   to common idioms (e.g. passing .data(), .data() + .size() from a null string). (#3615)
+- PlotHistogram: Fixed zero-line position when manually specifying min<0 and max>0. (#4349) [@filippocrocchini]
 - IO: Added 'io.WantCaptureMouseUnlessPopupClose' alternative to `io.WantCaptureMouse'. (#4480)
   This allows apps to receive the click on void when that click is used to close popup (by default,
   clicking on a void when a popup is open will close the popup but not release io.WantCaptureMouse).

imgui_demo.cpp

@@ -1561,16 +1561,17 @@ static void ShowDemoWindowWidgets()
     }
 
     // Plot/Graph widgets are not very good.
-    // Consider writing your own, or using a third-party one, see:
-    // - ImPlot https://github.com/epezent/implot
-    // - others https://github.com/ocornut/imgui/wiki/Useful-Extensions
+    // Consider using a third-party library such as ImPlot: https://github.com/epezent/implot
+    // (see others https://github.com/ocornut/imgui/wiki/Useful-Extensions)
     if (ImGui::TreeNode("Plots Widgets"))
     {
         static bool animate = true;
         ImGui::Checkbox("Animate", &animate);
 
+        // Plot as lines and plot as histogram
         static float arr[] = { 0.6f, 0.1f, 1.0f, 0.5f, 0.92f, 0.1f, 0.2f };
         ImGui::PlotLines("Frame Times", arr, IM_ARRAYSIZE(arr));
+        ImGui::PlotHistogram("Histogram", arr, IM_ARRAYSIZE(arr), 0, NULL, 0.0f, 1.0f, ImVec2(0, 80.0f));
 
         // Fill an array of contiguous float values to plot
         // Tip: If your float aren't contiguous but part of a structure, you can pass a pointer to your first float
@@ -1600,7 +1601,6 @@ static void ShowDemoWindowWidgets()
             sprintf(overlay, "avg %f", average);
             ImGui::PlotLines("Lines", values, IM_ARRAYSIZE(values), values_offset, overlay, -1.0f, 1.0f, ImVec2(0, 80.0f));
         }
-        ImGui::PlotHistogram("Histogram", arr, IM_ARRAYSIZE(arr), 0, NULL, 0.0f, 1.0f, ImVec2(0, 80.0f));
 
         // Use functions to generate output
         // FIXME: This is rather awkward because current plot API only pass in indices.

imgui_widgets.cpp

@@ -6471,7 +6471,7 @@ int ImGui::PlotEx(ImGuiPlotType plot_type, const char* label, float (*values_get
         float v0 = values_getter(data, (0 + values_offset) % values_count);
         float t0 = 0.0f;
         ImVec2 tp0 = ImVec2( t0, 1.0f - ImSaturate((v0 - scale_min) * inv_scale) );                       // Point in the normalized space of our target rectangle
-        float histogram_zero_line_t = (scale_min * scale_max < 0.0f) ? (-scale_min * inv_scale) : (scale_min < 0.0f ? 0.0f : 1.0f);   // Where does the zero line stands
+        float histogram_zero_line_t = (scale_min * scale_max < 0.0f) ? (1 + scale_min * inv_scale) : (scale_min < 0.0f ? 0.0f : 1.0f);   // Where does the zero line stands
 
         const ImU32 col_base = GetColorU32((plot_type == ImGuiPlotType_Lines) ? ImGuiCol_PlotLines : ImGuiCol_PlotHistogram);
         const ImU32 col_hovered = GetColorU32((plot_type == ImGuiPlotType_Lines) ? ImGuiCol_PlotLinesHovered : ImGuiCol_PlotHistogramHovered);