It will be a new module because it will have a new dependency on the Employee module. The existing module doesn't have that dependency but resources (the base module) can be used there. We can then abstract resource calendars from the Employee module.
We will add histogram on the new module in the next stage. For the existing module we will remove the tabpanel and put the Gantt in the same view as the Histogram like you requested.
If you wish all three combined, the new module will fit your requirements.