Weather Forecast - °C unit not displayed

Hi,

I'm skinning the Weather Forecast plugin, and regardless that the units parameter is set to a valid png image, it is not displayed, meaning that the temperature is displayed as +5 instead of +5°C. (°C and °F is in the png.) In fact this is the case with all the Weather Forecast skins.

Where shall I set in the plugin to display the temp unit?

Thanks,
szekelya