Extracting data from matlab graph
The data that is used to create the curves in the plot are stored in the axes object of the figure that contains the plot. You can obtain the underlying data by retrieving the axes' xdata and ydata properties. For example:

h_axes = gca;
x_data = get(h_axes, 'xdata');
y_data = get(h_axes, 'ydata');

After extracting the data you can use interpolation (see e.g. interp1())to find the value that you are looking for.

Is this what you meant? If not, please let me know and I'll try to explain better...