Feature: Add row and column offset for heatmaps

[ACvanWyk]

Thank you for the amazing library.

I have a use case where I store and add data in a circular buffer which is used to draw a heatmap. The circular buffer is preallocated and once it is full we start replacing data from the start.
To avoid having to shift and copy around data I suggest we add a column and row offset to the heatmap function which will shift the rows and columns based on the passed in values

[brenocq]

Hi @ACvanWyk! Since #519 was merged, PlotHeatmap now support Offset/Stride. For your application, you could store the data as ColMajor, and use the offset to plot. You would need to render two PlotHeatmap, one with offset 0, and the other with offset > 0 (depending on where the split of old/new data is in the circular buffer)

// Example: buffer_size=100, write_pos=30 (newer data at indices 0-29)
ImPlotSpec spec;
spec.Offset = write_pos;  // Start at index 30 (oldest data)
spec.Flags = ImPlotHeatmapFlags_ColMajor;
ImPlot::PlotHeatmap("##old", buffer, rows, buffer_size - write_pos, ...bounds_old..., spec);

spec.Offset = 0;  // Start at index 0 (newest data)
spec.Flags = ImPlotHeatmapFlags_ColMajor;
ImPlot::PlotHeatmap("##new", buffer, rows, write_pos, ...bounds_new..., spec);

Let me know if this works for you! I'm a bit hesitant to add offset per row/col because it will make the PlotHeatmap function bloated with arguments that most users won't use.