I need to get data from a pretty simple yet not really common type of plot (graph). If you take a look at this image, you will notice that the Y axis is irrelevant and that curve has to take its place.

The actual data I need is for each 'X'(l) read value of t(l).
I've already tried UN-SCAN-IT, Engauge, Digitizeit etc. and none of them seems to support curves as an axis – or maybe I'm going about this issue the wrong way.