Help with HW please

Hello, I need help conducting a program I was assigned. I am on the last last part but I do not know how to do it. Basically I graphed a derivative with its time, and now I need to do a second series on the same graph but all the values above 1500 need to be in read (just like ExampleGiven). I attached two pictures, one of what the graph needs to look like and the other is what I have at the moment. I just do not know what I am missing. Anything can help please and thank you.

clear all
close all

fid=fopen("HW10Data.txt");
scan = textscan(fid, '%n %n %n', 'Delimiter', 'tab', 'headerlines', 1);
Time= cell2mat(scan(1));
Altitude = cell2mat(scan(2));

for i = 2:22624    
    y(i) = Altitude(i,1)-Altitude(i-1,1);
    x(i) = Time(i,1)-Time(i-1,1);
    dte(i) = y(i)/x(i);
end

 figure(1)
 hold on
 plot(Time,dte, 'color',[0,0,0])
 xlabel('Time')
 ylabel('Altitude(Derivative)')

 for e = 1:22624
     
     if dte(e) > 1500
         BigNums(e) = dte(e);  
     else
         BigNums(e) = 0;
     end
     
 end

figure(1)
plot(Time(1:1500), BigNums(1:1500), 'Color', [0,0,0])
plot(Time(1500:end), BigNums(1500:end), 'Color', 'r')

Answer 1

[darova]

See this witty trick

x = 0:.1:20;
y = sin(x);
y1 = y;
y1(y1<0.5) = nan;
plot(x,y,x,y1,'.-r')

result