The following code is a modification of Wolfram code I found under ClickPane Help. I want to convert from Dynamic to Manipulate to make it easier for me to add various controls. I do want to keep the ClickPane formulation. Thanks.
DynamicModule[{f = {}},
ClickPane[
Dynamic@Plot[f, {t, 0, 10}, PlotRange -> 5, Frame -> True],
(AppendTo[f, Exp[-(t - #[[1]])]*#[[2]]]) &]]