If you have created a new drawing and haven't saved it yet, then the "save" symbol will remain grayed-out and cannot be selected. In this case, select 'save as', in order to be able to give your drawing a name. Once saved, the "save" function  is activated and you can save your drawing as the same name as already used.  If you want to save your distance-time diagram in a modfied version with a new name, select 'save as' again.