Algorythm for the interpolation?

Hi I have now managed to plot various points on a map and interpolate with ordinary kriging. However, my data does not look anything like it should. Do I need to use an algorithm to fill in the missing data? Does anyone know how the RCWIP2 model does this?

My data looks like this:

and it should look like this:

https://www.researchgate.net/figure/Global-mean-annual-average-leaf-water-d-18-O-and-d-2-H-isoscapes-for-the-sites-of_fig1_226462314

Does anyone have an idea where to get an algorythm (open source) for this? Or is it possible to create a better interpolation using the GNIP data (without an algorithm)?