getLastYAxis = function getLastYAxis(index) {
          if (typeof yaxis[index] !== 'undefined') {
            w.config.yaxis[index].min = yaxis[index].min;
            w.config.yaxis[index].max = yaxis[index].max;
          }
        }
This function reverts the yaxis and xaxis min/max values to what it was when the chart was defined.
This function fixes an important bug where a user might load a new series after zooming in/out of previous series which resulted in wrong min/max
Also, this should never be called internally on zoom/pan - the reset should only happen when user calls the updateSeries() function externally
The function also accepts an object {xaxis, yaxis} which when present is set as the new xaxis/yaxis