By default, the HistogramBorderSize property is 3. Use the HistogramBorderSize property to specify the width of the frame being shown in the histogram, only when curves are shown ( the HistogramPattern property is not a predefined value, 256, 512, 1024, and so son ). Use the HistogramBorderColor property to change the color for the frame being shown in the histogram. For instance, if the HistogramPattern property points to a predefined value, the color for the frame in the histogram is automatically determined by the HistogramColor property. 

The following screen shot shows the histogram of bars using different size and color for the frame: