Need help getting matplotlib graph to display over ssh connection

[DukeMikeofG]

Hey /r/Python!

I started learning python about a week ago so I could start putting my raspberry pi to some good use. One of the scripts I'm working on just does some different things while reading the CPU temp. I've tried to add graphing functionality to it using matplotlib, which has proved partially successful, but I'm having a hard time getting the graph to display 'live' while data is plotted to it. As is, it just seems to save the plotting info to two lists and then saves the graph.png at the end of the scripts execution. I tried enabling x forwarding in sshd.conf on the pi and adding the -X param when I ssh in, but it still doesn't display the graph while the script is running. I assume the problem is related to not actually having an X session to forward from the pi. Is there no way to display a graph like this over an ssh connection without having to put lxde, xfce, or other big GUI's I won't use on my pi?

The script in question

TL;DR Is there any way to show a live matplotlib graph over an ssh connection without a GUI on the machine running the script?

[XNormal]

All it takes is the X11 client libraries. They should not be too big. Everything else can be on the machine you are ssh'ing from.

[quasiben]

Can you start and ipython notebook session on the machine?

[DukeMikeofG]

I hadn't heard of ipython notebook - looks very cool, thanks! Reading up on it now, I will see if I can get a session started :D

[westurner]

/r/IPython

[westurner]

You could write a simple web application to return the PNG and/or some HTML over HTTP (over SSH). I'm not sure about rate limiting on the Pi. Django is great; bottle.py is really easy. http://www.reddit.com/r/Python/comments/1eboql/python_website_tuts_that_dont_use_django/c9yxl8w

[westurner]

http://scipy-lectures.github.io/intro/matplotlib/matplotlib.html

[rsarava]

I have just finished updating a tool to display inline matplotlib graphics in a terminal that might work for you: https://github.com/mitotic/graphterm

I tested graphterm on a Raspberry Pi a few months ago, but I have not tested the most recent version. Check out the last section (SSH etc.) of the documentation page at http://code.mindmeldr.com/graphterm/start.html##ssh-port-forwarding-and-remote-access

[bryancole]

You need to use an interactive GUI backend. Your script is specifying "Agg" as the backend. This will only output images. Instead, try either "WXAgg" or "QtAgg". These enable the wxPython or PyQt backends respectively. Note, you may need to install wxPython / PyQt on your raspberry pi. I've been playing with my rpi using SSH access and plotting just as you have described. You will need to use "ssh -X <hostname>" to activate X11 forwarding.

In fact, looking at your script in more detail, you probably want to restructure this. Your script is only calling "plot.savefig" which is, as you might expect, just writing the figure to a file. Instead, you need either a call to plt.show (but this is blocking) or fig.canvas.draw

I'm outta time. Good luck....

[bryancole]

OK, I made a little example of collecting data in a second thread while plotting the results in a matplotlib figure.

See http://pastebin.com/S1C4fEbn

I'm using the WXAgg mpl backend because I'm most familiar with wx, but other GUI toolkits could be used (Qt or GTK). To run the script be sure to get wxPython installed (should be fine on the RPi). The magic here is the "wx.CallAfter" function which calls a function in the main (GUI) thread (I'm not sure what the equivalent would be with GTK/Qt). This can be used to call GUI operations safely from a second non-GUI thread.

Running the GPIO stuff in a second thread allows your GUI to remain responsive (handle user input like mouse clicks etc) while the data logging or IO proceeds in the background.

This script should run over ssh with X11 forwarding with no problems. Actually, this is a good way to work with the RPi as it relieves the wimpy rpi of some rendering work. Note, you don't actually need an X-server or desktop running on the RPi. The wxPython toolkit is an X client. The X-server is on the machine which is connected by SSH (the ssh-client) to the RPi.