How to automatically refresh the graph when i open a second file to be processed?

Difficult to say without code, but if you are creating the entire graph dynamically, then you would need to dispose of the original graph first OR edit the original graph ie:

Chart tmpChart = new Chart();
tmpChart.Width = 100;
tmpChart.Height = 100;
tmpChart.Location = new Point(100, 100);

this.Controls.Add(tmpChart);

This code will add chart on top of chart each time I use it to add a chart.
However:

Chart tmpChart;
tmpChart = (Chart)this.Controls["TempChart"];
if (tmpChart == null) { tmpChart = new Chart(); }
tmpChart.Width = 100;
tmpChart.Height = 100;
tmpChart.Location = new Point(100, 100);
tmpChart.Name = "TempChart";

this.Controls.Add(tmpChart);

This code will replace the information on the same chart with the new information. This sort of method works for basically all controls in c#. hope this is somewhere near what you are asking.