Make your MACD histogram one color above zero and a different color below zero.
Larry Nelson commented
I had planned to enter this very same suggestion. Allow the user to pick to the colors of the histogram above/below zero, or make them green (above 0) and red (below 0).
William Orlowski commented
You can already do this with StockCharts.com and this program is a much more porwerful graphing program than VV.