In the processing toolbox (Processing > Toolbox) you can find a pile of suitable tools (search for "regression" and "sample").
You could first create a raster from the regression analysis (SAGA' tools "Polynomial regression" or "Regression kriging" rsp.)
Results in some raster like
You may sample this with your ...
The R2 value is not shown on the chart by default (I don't know if there is a way to change this behaviour).
It is displayed if you hover the mouse over the legend field for the line, on the displayed chart, or click on that same section.
Here is an example, using different code (R2 section highlighted in red):
Yes i want the single number to predict the value everywhere but the grid is not constant. Let's say for example, i have 3 single values and 3 grids, with 4 cell each one. All the grids have the same extension. Given a spatial location (that is, given a single cell in a certain position), i'll have 3 dependant values, one for each grid, in that cell: each ...