Broken pipe issue - octave

Question

I'm trying to learn machine learning using Octave.
OS: Mac, High Sierra 10.13.3
GNU Octave version: 3.8.0
t = [0: 0.01: 0.98];
y1 = sin(2*pi*4*t);
y2 = sin(2*pi*4*t);
plot(t, y1);
hold on;
plot(t, y2, 'r');
figure(1); plot(t,y1);
The first time I tried ran the last line I got: "Qt terminal communication error: select() error 9 Bad file descriptor"
After this I got a broken pipe error, I thought I need to plot both the graphs again before trying "figure(1); plot(y,t1);". But, it didn't work.
Checked whois and printed the values of y1, y2 and t; they do exist. So, my questions are:
Why am I not able to plot the graphs?
How do I solve broken pipe error?
How to solve the Qt terminal communication error?

Answer 1

Coursera's Machine Learning class is still using this version of Octave (3.8.0)
Here is what helped me:
run the following the os x terminal:
brew uninstall gnuplot
brew install gnuplot --with-qt
then create a file in your home directory called .octaverc and add the following to it:
setenv("GNUTERM","qt")
Relaunch octave-cli
Good Luck!

Answer 2

simply close the terminal and relaunch it , and this fixes the issue.
This occurred to me as I tried quitting the gnu-plot instead of using close command causing the pipe linked to gnu be broken.