How do I display negative values through geostats?

Hello, I'm busy mapping temperatures for locations around the world and in some cases the value is negative. Unfortunately negative values are not displayed in the map through the geostats command, so I was wondering how to enable them? I've tried setting the marker sizes so that it is the same for all values: 1010 But this only makes positive values the same size, negative values are still missing... Any ideas? Thank you! Andrew