While working on #1003 I noticed that when the axis limits are set to the exact dimensions of the heatmap the black lines that normally outline the plot (axis lines) are missing on the top left suggesting it's off by 1 pixel. There's also a row of white pixels on the bottom which could be related (and/or an anti-aliasing rendering artifact). Since there's no white column on the right I question if the bitmap is also being accidentally rendered 1px too wide.

var plt = new ScottPlot.Plot(400, 300);
var hm = plt.AddHeatmap(data, lockScales: false);
plt.AddColorbar(hm.Colormap);
plt.SetAxisLimits(0, data.GetLength(1), 0, data.GetLength(0));