help generate chirp signal

hi, i want to generate a chirp signal with frequency going from 10Khz to 100Khz. Currently i am using the following code to generate chirp where frequency goes from 100 to 500hz. But as you can see the graph, the plot is not a continuous plot, i want a continuous up chirp. also, if i change the values f1, f2 in the code below the output is not correct.

code:

Fs=1000;

tf=2;

t=0:1/Fs:tf-1/Fs;

f1=100;

f2=400;

semi_t=0:1/Fs:(tf/2-1/Fs);

sl=2*(f2-f1/2);

f1=f1*semi_t+(sl.*semi_t/2);

f2=f1(end)+f2*semi_t-sl.*semi_t/2;

f=[f1 f2];

y=1.33*cos(2*pi*f.*t);

plot(t,y)