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  • Writer's pictureJack Orenstein

Generating graphs from the marcel command line

I was working on a project in which I needed to munge some data, producing a stream of (x, y) tuples, and then graph the data. I had always resorted to python scripts and matplotlib when creating a graph, but I thought it should be simple to generate graphs from marcel. First I'll show you the verbose way of doing it. Then I'll show you how to use marcel's parameterized pipelines to do something much more usable.

The graphing function is easy. This function uses matplotlib to create a graph of (x, y) points, connecting adjacent points with a straight line. Note that there are two parameters, a list of x values and a list of y values.

import matplotlib.pyplot as plt

def plot(xlist, ylist):
    plt.plot(x, y)

I can put arbitrary code into my marcel startup script (which is typically in ~/.config/marcel/, so I just put this code (the import and the function) in there.

But the problem is that I was producing a stream of (x, y) values, and I needed to separate the x values into one list, and the y values into another. That's easy in marcel. For example, suppose I want to plot sin(x) for x = 0 .. 360 (degrees). Generating the point coordinates can be done as follows:

import math
gen 361 | map (x: (x, math.sin(math.radians(x))))

This yields a stream of 361 pairs:

(0, 0.0)
(1, 0.01745240643728351)
(2, 0.03489949670250097)
(3, 0.052335956242943835)
(4, 0.0697564737441253)

I can use the reduction operator, red, to concatenate all the x values into a list, and all the y values into a list:

gen 361 \
| map (x: (x, math.sin(math.radians(x)))) \
| red concat concat

concat is a marcel reduction function, which accumulates elements into a list. So we are accumulating two lists, one for each position of the incoming pairs.

Next, I simply invoke the graphing function, plot:

gen 361 \
| map (x: (x, math.sin(math.radians(x)))) \
| red concat concat \
| map (xlist, ylist: plot(xlist, ylist))

The command pops up this graph:

"red ... | map ..." is kind of a lot to type every time I want a graph. But I can put all that in a pipeline, stored in a variable -- basically a function with no arguments.

graph = (| red concat concat \
         | map (xlist, ylist: plot(xlist, ylist))

(| ... |) delimits the pipeline. Inside, there is the code that converts the (x, y) stream into lists of x and y values, and the invocation of the graphing function. To use the pipeline:

gen 361 | map (x: (x, math.sin(math.radians(x)))) | graph

And now, anytime I have a stream of (x, y) values, I can use this graph pipeline to create a graph.

Taking things farther: The graph pipeline is fine for quick and dirty graphing. But if I want to show the graph to people, I need a few additional things:

  • A title.

  • Labels for the axes.

  • The image needs to be stored in a file.

This is all easy to add. First, I extend the plot() function that I

wrote, to place titles and labels on the graph, and to save the graph in a file:

def plot(title, xlabel, ylabel, filename, x, y):
    if title:
    if xlabel:
    if ylabel:
    plt.plot(x, y)
    if filename:

Second, I modify the graph pipeline. Stored pipelines can have parameters, allowing you to pass values on the command line. So:

graph = (| title, xlabel, ylabel, filename: \
           red concat concat \
         | plot(title, xlabel, ylabel, filename, xlist, ylist) \

I've added the parameters title, xlabel, ylabel, and filename to the pipeline, which passes them to the plot function.

So now I can get a graph with a title and labels, simply by passing those pieces of text to the graph pipeline:

gen 361 \
| map (x: (x, math.sin(math.radians(x)))) \
| graph 'y = sin(x)', 'x(degrees)' 'sin(x)'

And here is the more presentable graph:

Notice that I didn't specify a value for the filename argument. That means that the filename argument will take on a value of None. The plot function tests the filename, and if it is None, then the graph is not saved, (it's still displayed though).

Because missing arguments are set to None, I can always leave off the title and axis labels, and still get a quick and dirty graph.

What if you want to specify a filename, but not a title or axis labels? Pipeline parameters can be referenced using long option names, e.g.

gen 361 \
| map (x: (x, math.sin(math.radians(x)))) \
| graph --filename sin.png

If you like this graph pipeline, you can put it in your startup script, and then it's always there.

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