Please mind that RadHeatMap is quite different than RadGridView. It doesn't have notion for rows and columns. In fact it is a matrix-like control that encode colored values along two axes. These values (cells) can not be styled independently as the control uses a bitmap-based rendering i.e. as I previously mentioned renders a WriteableBitmap image. You can easily see this if you inspect the control's visual tree with XAML Spy for example. So practically there is no way that you can have Grid-like styles applied on an image. What we can suggest you is that you create your own custom control and plug it to the HeatMapColorGrid
template. What this control should do is calculate the rows count and columns count of the HeatMap and provide some logic for drawing a horizontal line after a specified row position.
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