Reconstruction of Data from a Chart or Graph

I have here several charts of driving cycles. These are standard plots derived from real traffic data, of velocity versus time. Unfortunately I cannot find the data anywhere. So I hatched a plan: maybe there’s software that will reconstruct data from a graph or chart? Does anyone know? If not, I’ll just write it myself and open source it. It seems like a generally useful thing in engineering and science. It deserves an application (maybe even a web application).

Unified California Drivecycle