Allow make operations with fields

[CarusoGeo]

Allow to create operations between fields, such as: average, sum. Example: put the average of the values in each grouped month on the x-axis. And in addition allow to visualize this with using another field to separate the bars into different classes.

[patrickb]

@CarusoGeo Can you explain in a bit more detail? This might be a case where an image/picture of what you're after would help us understand better.