Well, if you need to get the upper left row/column and lower right row/column indices, you may do it via using the following attributes, see the sample lines of code for your reference:
int UpperLeftRow = chart.ChartObject.UpperLeftRow;
int UpperLeftColumn = chart.ChartObject.UpperLeftColumn;
int LowerRightRow = chart.ChartObject.LowerRightRow;
int LowerRightColumn = chart.ChartObject.LowerRightColumn;
string area1 = CellsHelper.ColumnIndexToName(UpperLeftColumn) + UpperLeftRow + 1":" +
CellsHelper.ColumnIndexToName(LowerRightColumn) + LowerRightRow +1;
Let us know if you have some different issue or requirements.