How can I plot the derived function of cubic spline interpolation?

Hello, I want to ask how I can plot the derived function of cubic spline interpolation? I show how I did it in my code. The error was:

*Error using plot *Invalid second data argument *Error in function_MTU4000_Real (line 90) *plot1=plot(x1,Speed1,'b');

 %calculation of lifting of intake valve (approximation spline function)

x1=0.0:0.1:202.1;
y1=xlsread('Steuerzeiten_Schrittweite_MTU4000.xlsx',1,'D2:D2023');
Lifting1=spline(x1,y1);
x2=202.1:0.1:701.9;
Lifting2=0*x2;
x3=702.0:0.1:720.0;
y3=xlsread('Steuerzeiten_Schrittweite_MTU4000.xlsx',1,'D7022:D7202');
Lifting3=spline(x3,y3);
%calculation and plot of speed intake
figure(2);hold on; grid on;
Speed1=fnder(Lifting1);
plot1=plot(x1,Speed1,'b');
Speed2=Lifting2;
plot2=plot(x2,Speed2,'b');
Speed3=fnder(Lifting3);
plot3=plot(x3,Speed3,'b');
hold off
legend([plot1,plot2,plot3],'Intake')
set(gca,'XTickLabel',{'OT','90','UT','270','ZOT','450','UT','630','OT'});
title('Intake Valve Speed')
xlabel('Crank Angle [°]')
ylabel('Speed [m/°]')

[KSSV]

Try to print the values of Speed1....does it have any numbers? We cannot check the code unless the xlsx file we have in hand.

[OK]

Thank you very much for your answer! Ok, i attached the file as text, because i can't attach excel, but you can bring these values to a excel file and rename it the same. Besides if i write the code below, i get 'NaN' in the text file 'Speed1.txt'. So i think that something is wrong with Speed1. I hope you can help me to fix it.

%writing Speed1 to Speed1 text file
fileID=fopen('Speed1.txt','w');
formatSpec='%d\r\n';
Sp1=str2double(Speed1);
table=[Sp1]';
fprintf(fileID,formatSpec,table);
fclose(fileID);