How to extract information from plot images?
In the general sense this is most definitely a non-trivial problem and any full featured solution will require a considerable degree of work.

Success will probably be dependent on how much you constrain your problem.
Monochrome will be easier than color.
Limited font, font size, line and marker styles will help.

The more constrained, the more chance of success.

For line detection, such as axes, you might want to consider the Hough and Radon transforms.

Here are some stackexchange questions covering some of the topics you might want to address, sadly no bargraphs, some have a manual component which you might think how to automate. They are a good starting point to review some of the difficulties involved in the problem.

Selecting a curve:
https://mathematica.stackexchange.com/questions/44355/how-to-make-a-curve-selectable-from-a-scaned-image-and-convert-it-to-a-list-of-c

Datapoints and Axes:
https://mathematica.stackexchange.com/questions/1524/recovering-data-points-from-an-image

Scatter:
https://mathematica.stackexchange.com/questions/26356/how-do-i-find-the-coordinates-of-points-in-this-image

Datapoints:
https://mathematica.stackexchange.com/questions/3831/how-can-i-extract-data-points-from-a-black-and-white-image

https://mathematica.stackexchange.com/questions/14444/obtain-data-points-from-a-graph-on-an-image-without-axes