Hi Samantha Clayton:
Since I do not know your code I can only demonstrate in the following example how to draw a vertical and a horizontal line into a contourf-plot (assuming you mean something like that).
Z = peaks(20);
ah = axes;
h = contourf(Z,10,'Parent',ah);
plot(ah,[0 25],[10 10],'white')
plot(ah,[10 10],[0 25],'white')
ah.XLim = [2 20];
ah.YLim = [2 20];
There is a HeatMap-Command in Bioinformatics toolbox, so I do not know whether same strategy is working for that.