19. Graphics and plotting

Every workflow can have one or more special units, which do not influence control flow but demonstrate other units’ internal state. such units are called “plotters” and should inherit from Plotter. Normally, they use Matplotlib to draw plots, images and graphs and always work in a separate process, probably on remote machine, called “graphics client”.

Here is the diagram showing how plotters work:


Each plotter does initial data processing, taking only needed data from linked properties. Then it is pickle.dumps()-ed and sent over the network through ZeroMQ EPGM endpoint. It means that there can be any number of clients receiving and drawing plots on different machines. The work is done by GraphicsServer class. The graphics client listens to broadcast messages in GraphicsClient, deserializing received units and calling redraw() on each. Thanks to the way Matplotlib plotting works, units do not need to recreate everything from scratch, though they should be ready to.

Normally, one graphics client instance is launched during VELES startup, but can be disabled with --no-graphics-client. To launch a graphics client manually, execute:

python3 veles/graphics_client.py -e <endpoint>

where <endpoint> is the broadcast ZeroMQ address, printed in startup logs or viewable through the web status page.

You may specify WebAgg Matplotlib backend (-b option here, or -p in command line), so that plots will be accessible through the local network or the internet.

There is a feature to write each plot into a PDF file. To activate PDf mode, send USR2 signal to graphics_client.py process, something like:

killall -SIGUSR2 python3

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