Initialise grouped keys with 0

If you were trying to group a bunch of values down to a key that did not
already exist you would end up with NaN as it would try and add all
values to undefined.

We now check whether it is undefined and set it to 0 as appropriate.
Fix table rendering where X axis is label

If we are making a groupped query and do not have time as the X axis,
then we do not want to try and collapse the data.

This is the case in tables where the first column (x axis) is a label
against the sum of a particular field.

The line and stacked graph tests had to change as the data was expecting
to be collapsed but it needed a date as the x axis to make this happen.