Insert a mean trendline or best fit line

Hi,

I'm currently working on a project that compares infiltration rates on soils on grass and in forest. I have a dataset for both soils and I plotted them in a scatter (see attachment). I would now like to create some kind of mean line, a best fit line through the data points of grass and soil in order to make a better comparison. Can anyone help me? I tried the exponential and moving avarage, but they do not quite fit the data well enough; I would really like a smooth best fit line. If this can't be done in Excel, does anyone know a good program of software that can?

Thanks already!