Vera does not store sensor values history. However, there is a great free vera plugin called DataMine that handles that for you.
If you install the DataMine plugin and configure it properly to store your sensor values, ImperiHome will detect it and generate proper charts.
Please check that you properly configure DataMine for graphing by checking graph visualization in DataMine web interface : if web interface graphs do not work, then ImperiHome won't be able to display graphs either.
A detailed explanation doc about Datmine itself : http://code.mios.com/trac/mios_datamine/raw-attachment/wiki/WikiStart/dataMine%20Overview.pdf
NB : Be careful to add the right device variables to DataMine history. For example, for energy meters, you have to activate DataMine on the "Watts" state variable.