Double Interpolation using 2 Input Variables

Code:

IDF = readtable('IDF_Data.xlsx')
D   = 9

if D ~= IDF.Var1
    In = interp1(IDF.Var1, IDF.R2, D);
end

disp(In)

In = 6.5

Given a table of data for rainfall intensity, write a script that receives as input parameter a specific storm duration (D) minutes in the range of 0 to 1440 minutes, and return period (R) years in the range of 2 to 100 years. The output should be rainfall intensity (I) in/hr.

If the user inputs a value for R and D that are not in the table, how do you write a script that will double interpolate to find the value for I?

Example: R = 11, D = 17, Disp(I)

XL table: Matlab table:

With what I have now, Matlab reads row 1 as part of the output data. How will Matlab read row 1 as input values?