@freemo The style of chart you're looking for is an Ashby chart. With some google-foo, the closest flavor of Ashby chart to what you want I could find publicly available is this:
@freemo The relation between diffusivity (a) and specific heat (Cp) is:
a = k / (rho * Cp)
where k is conductivity, rho is density. So the X axis in this chart is inversely proportional to Cp, but also proportional to k (the y-axis!). The 45 degree dashed lines in the chart are isometric to volumetric specific heat.
@freemo Source: slide 14 of this (sorry for the spammy ad site - was the closest to original source I could find. I don't have a copy of Ashby's book handy): https://www.scribd.com/document/337935389/Ashby-Charts
If I found it useful the company I run (or its non-profit depend on where its appropriate) could always probably afford a license. That said I do have a very strong preference for open-source tools that are standards based.