How to collect Y values in graph plot in MATLAB APP Designer other than indexing?

I keep getting the error: "Array indices must be positive integers or logical values." whenever I try to run my code.

I have a plot of delta Tc (the change in temperature for the refrigerant) against its flow rate. The problem is I would like to be able to collect the specific delta Tc values just by changing the discrete knob for the flowrate. The option I have found so far was indexing, however it does not accept values that are not integers. Can someone help me?

function CalculateButtonPushed(app, event)
            t = readtable("flowrates.xlsx","Sheet",1);
            Hrate = str2num(app.HotFlowrateLminKnob.Value);
            Hin = str2num(app.HotFluidTemperatureInCListBox.Value);            
            flowrateC = str2double(app.ColdFlowrateLminKnob_2.Value);
            specific_heat = 4200;
    if Hrate == 1 && Hin == 60
                    x = table2array(t(:,"Fc1Lm60H"));
                    y = table2array(t(:,"delta_Tc1"));
                    plot(app.UIAxes,x,y);
                    delta_TC = y(flowrateC);
                    app.deltaTcEditField.Value = delta_TC;
                    heat_transferred = (flowrateC/60)*specific_heat*delta_TC;
                    app.HeattransferredJoulesEditField.Value = heat_transferred;
                    app.Lamp.Color = "g";

[Joao Gocan]

Already solved it: https://uk.mathworks.com/matlabcentral/answers/265065-question-about-finding-y-value-for-certain-x-value-of-a-plot#answer_207309