The Shape of Code

There are two techniques for specifying the display information contained in any graph within a pdf file. Raster graphics specifies (e.g., jpeg and png formats) the location and color of pixels, and vector graphics specifies (e.g., pdf and svg) the x/y coordinates of shapes/colors on the page. With vector graphics it’s possible to calculate the graph-axis relative positions of displayed symbols using the page relative x/y positions of each symbol (denoting a measurement point), the graph axis and axis tick marks/values; something I first did out in 2013. My Evidence-base software engineering book contains 10+ datasets that have been manually extracted from vector graphics within pdf files, i.e., by writing a graph specific awk script (a summary of the processes and tools used).

Can coding agents be used to implement a program detects and extracts the x/y coordinates (axis relative) of data points in any graphs, implemented using vector graphics, contained in a pdf file?

Do LLMs know of any tools that already do this? No.

This weekend I attended the Reinvent the Wheel: A Tools for Science Hack, where attendees were given credits for a month of Claude Max (these days I don’t write enough code to make it worth spending more than $20 per month on subscriptions). This was a perfect opportunity, and motivation, to create a specification of the data point x/y coordinate extraction process, and then to attempt to get a coding agent to generate a python program that extracted this data from any pdf file (whose plots were created using vector graphics).

Data points within a graph are drawn using a variety of symbols, and the symbols are drawn using a variety of techniques. For instances, a circle might be drawn using a single internal pdf instruction, drawing four quadrants using Bézier curves, or even eight Bézier curves, or who knows what (it’s very frustrating).

Then there are the tick marks along the axis, some of which are labelled with numeric values, don’t always start/finish at the ends of the axis, and denote a scale that could be linear or logarithmic.

My plan was to specify what was required, briefly describe the internal organization of a pdf file, provide a pdf file containing a single graph and the corresponding extracted data, as a csv file, and see what happens.

The start of my conversation with Claude Opus 5.5 follows:

Write a python program that takes as input a pdf file and
outputs the x/y coordinates of individual data points displayed
using vector graphics.
 
The vector graphics, as specified by internal pdf instructions,
used to display these data points appear on lines bracketed by
lines that just contain the words "stream" and "endstream"
 
The data points appear within a graph displayed on a page.
This graph contains an x and y axis and may contain other
unrelated lines and text within the plot's x/y axis.
 
The required x/y data point coordinates must be relative to
the values displayed on the plot's x and y axis.
Each x and y axis is drawn as one long line and short tick
marks for particular axis values.
An internal instruction is used to display a numeric value
close to its associated tick mark.
 
Various symbols may be used to display each data point.
For instance, in the attached pdf file a data point is denoted
by a circle created by joining four bezier curves.
The required x/y coordinate is at the center of the
approximate circle drawn by these four bezier curves.
 
All data point coordinates must be output, even data points
having the same x/y coordinates as other data points.
 
The attached file schump.tgz contains a pdf file and
a csv file containing the internal x/y positions of the
data points and the corresponding graph relative x/y positions,
it also contains the page number and the color of the data points.
Each graph is numbered.

It took about 15 minutes to create a python program that extracted the information from the pdf file that matched the contents of the csv file. I was impressed.

The generated program was then extended one feature at a time by describing some possible characteristics of a graph, and attaching a pdf containing a graph containing those characteristics; for instance (another 15-20 minutes):

Extend the program to support data points displayed as a
diamond shape created by drawing four lines.
 
The attached file 53cfc681e8e2afa9feb5820f4886e3018005-009.pdf
is a pdf file. The data points are denoted by diamonds created
by drawing four short lines.

This one/two feature at a time iteration looped six or so times before I grouped the remaining features into one update. Two+ hours later Claude provided 4K+ LOC of python code that appeared to handle everything.

The most impressive behavior was spotting minor issues that I had not mentioned and supporting them, or asking if they should be supported.

The pdfgraph2csv GitHub repo contains the code + regression tests.

Is the information in the output csv files correct?

The x/y coordinates of the points in the graphs in my book are available to be checked against. I also plan to repeat the process using a coding agent from another vendor, and to compare the contents of the corresponding csv files. N-version programming, here we come.