You can modify the PlotArea’s appearance by using the Background and Stroke/StrokeThickness properties for the border of the plot area. The plot areas are referenced by using the row/column (the same as the elements in the grid).
The following sample shows how to modify the Plot Area appearance:
<Chart:ChartView.PlotAreas>
<!-- row=0 col=0 -->
<Chart:PlotArea Background="#10FF0000" Stroke="Red" />
<!-- row=1 col=0 -->
<Chart:PlotArea Row="1" Background="#1000FF00" />
<!-- row=0 col=1 -->
<Chart:PlotArea Column="1" Background="#100000FF" />
<!-- row=1 col=1 -->
<Chart:PlotArea Row="1" Column="1" Background="#10FFFF00"
Stroke="Yellow" />
</Chart:ChartView.PlotAreas>