How can I convert xy line-plots to textual data values?

Question

I want to reverse engineer a simple XY-line graph to scalar data values.

An example of such a plot is linked below, water temperature as a function of datetime. As far as I know, the raw data (sensor readings) and the processed numeric temperature values of this are not openly available on the internet, not in text format.

http://www.met.fu-berlin.de/de/daten/mevis-grafiken/TegelerSee/TEGSEE_Jahr_TT-WT.jpg

I think such tools are available as desktop applications (e.g. Neuralog) but those I know are nonfree, expensive, and require too much manual interaction.

What I want to do is process the same plot (linked above) again and again, say every two weeks. Automating the data cleanup (remove outliers, merge timeseries) seems doable to me.

(Maybe this been asked before by someone else on one of the other sites (software-recs, SO)...)

Answer 1

Here is a blog post that compares 16 graph digitizing tools, some free, some not. The suggestion is to use WebPlotDigitizer. The blog doesn't mention if any of the tools can be used to automate the process.

To read an image file and process the data, then you probably want to use a programming language. Python or R-digitize are good places to start.