How to find a value inside contourf plot?

I have built a contour map using contourf function like in the picture below.

Where x-axis is the change of temperature in A and y-axis is the change of temperature in B. Different color in that picture show different slope increase in percentage. I have also three black dots (i.e when temperature in A decrease 2 degree, at the same time temperature in B increase 1.89). My problem is: Each dot also has their own slope change value. I want to know the exact slope change value, not only by looking the range of color in the color bar. Thus, how can I calculate this slope change value for each dots?

Thank you very much for your help.

[Carl]

Hi Handriyanti. I might be misunderstanding your question. You mentioned that the "slope change value" corresponds to the range indicated by the colorbar (the z value). If you are creating that contour plot with 'contourf', you should already have the values that correspond to each point. That is, the value at some (x,y) will already be in the 'Z' matrix you passed to the 'contourf' function. There's no need to look at the colors.