Each image is a correlation network of frequent word stems from 18 chunks of English text: 10 from Twitter, 4 from news and 4 from blogs. Each node is a word stem that occurs at least lowfreq times across all chunks. Two nodes are joined when their counts across the 18 chunks have a correlation of at least corThreshold. The graphs were drawn with Rgraphviz.

Move the pointer over the image to change the parameters. Horizontal position sets corThreshold, from 0.840 to 0.910 in steps of 0.001. Vertical position sets lowfreq, from 7,500 to 8,500 in steps of 100. The sliders do the same, and either axis can be locked. The line below the image shows the current values, the number of nodes and edges, and the image file. There are 781 parameter combinations, which produce 553 distinct graphs.

The graphs above use a steps of 0.001 for corThreshold, which skips most intermediate graphs. The strip below holds lowfreq fixed and shows every distinct graph in a narrower range.

These 87 graphs below come from a variant of the text cleaning that replaces every non-letter character with a space. That turns “don’t” into “don t”, so “don” becomes one of the 52 most frequent stems in place of “feel”. And “t” is removed. Here lowfreq is fixed at 8,228, the only value that gives 52 nodes. Moving the pointer from left to right raises corThreshold from 0.870 to 0.9105 and removes one edge per step, from 545 edges down to 458.

Rebuilding the frustration_of_time correlation plot

The goal

In 2014, during the Coursera Data Science capstone, I made an error-prone word correlation network from the SwiftKey English text data and saved it as frustration_of_time.png because it looked pretty. And then I fixed the errors and moved on with the assignment. Only that low-resolution PNG survived. The goal of this project was to find the exact parameters that produced it, so the same plot could be regenerated as a vector image and printed at any size.

The plot is drawn by tm and Rgraphviz from a document-term matrix built from 18 text chunks (10 Twitter, 4 news, 4 blogs). Two parameters control what it shows:

• lowfreq (lf) drops rare word stems. A stem becomes a node only if it appears at least lf times across all 18 chunks.
• corThreshold (ct) drops weaker correlations. Two nodes are joined by an edge only if their counts across the chunks correlate at ct or higher.

The original has 52 nodes and somewhere around 460 to 520 edges.

What I looked for.

1. Swept both parameters. cap_week2/corplot_multigraph.R walks a grid of lf and ct values, works out which cells produce identical graphs, and renders one SVG per distinct graph in parallel. The first sweep (lf 7500 to 8500, ct 0.840 to 0.910) produced 553 distinct graphs in about 90 seconds, in out/.
2. Built a web scrubber to compare them. The images were converted to WebP and uploaded to WordPress, and web/scrubber.html lets you move the pointer over the image to scan through lf and ct, with sliders, axis locks and a live readout. Getting it to run inside WordPress took a separate build (web/scrubber_wordpress.html), because WordPress rewrites characters inside scripts.
3. Narrowed it to 52 nodes. Only one set of 52 words exists, so lf was effectively settled and only ct mattered. The original grid had skipped most edge counts, so fine sweeps with a ct step of 0.00001 filled in every distinct 52-node graph (out_lf8000_fine/, out_lf8000_fine_low/). Many were close, but a chain of Twitter words in the lower right (good, love, great, today, follow, thank) always hung off the side, where the original has it tucked inside.
4. Reproduced the 2014 text cleaning. The old notebook cap_week2/task_2.2.Rmd replaced every non-letter character with a space before building the matrix. cap_week2/build_checkpoint_2014clean.R rebuilds the matrix that way. It changes one of the 52 words: don replaces feel. That single word was enough to change the layout. The next section explains why.

Why “don” replaces “feel”

Stemming cuts words down to a common root so that different forms count as one word. “Feel”, “feels” and “feeling” all become feel; “really” becomes realli and “people” becomes peopl. The nodes in the plot are these stems, not whole words.

Before stemming, the text goes through several cleaning steps, and their order matters:

1. Stopword removal deletes very common words like “the”, “and” and “is”. tm’s English stopword list includes contractions such as “don’t”, “can’t” and “isn’t”, spelled with the apostrophe.
2. In the current cleaning, the apostrophe is still there when stopwords are removed, so “don’t” matches the list and disappears.
3. In the 2014 cleaning, every character that isn’t a letter was first replaced with a space. The apostrophe in “don’t” became a space, leaving two words, “don” and “t”. Neither is on the stopword list, so neither is removed. “t” is later dropped for being shorter than 3 letters, but “don” survives as a word of its own.

“Don’t” is extremely common, especially in tweets, so this adds a lot of don. In the 2014-cleaned data don appears 14,167 times, which puts it well inside the most frequent words. In the current data it appears only 344 times, from other uses of the word.

Why it’s feel that drops out: the plot keeps the 52 most frequent stems. In the current data, feel is number 52, with 8,093 occurrences, just above the 53rd word. When don joins the top of the list, every word below it moves down one place, and feel becomes number 53 and falls off. The cutoff for 52 words rises from 8,000 to 8,228, which is why the 2014-cleaned graphs use lf 8228.

None of the candidates is a pixel-perfect match yet, but when that lower right tail tucks in, it was as close as I could get.

Diminishing returns

These were tested and didn’t get any closer to the original:

• An older Graphviz. It turned out Rgraphviz has bundled Graphviz 2.28 since about 2012, so 2014 and today use the same layout engine.
• Word order. All 52 stems are plain lowercase, so no locale or sorting rule could reorder them.
• Plot size and device. Rendering at 10 inches, 12 inches, and the original’s 704 x 627 pixels, with both the Quartz and Cairo devices, changes the layout somewhat, but nothing came out closer than the 12-inch set.
• More edges or fewer. Graphs from 425 to 580 edges, beyond the likely range.
• Other cleaning variants. Keeping straight apostrophes, as a later notebook did, brings feel back. Keeping accented letters changes nothing.
• The 2014 stemmer. SnowballC 0.5.1 produces exactly the same matrix as today’s version.
• The 2014 tm. tm 0.6 installs but fails on R 4.6, so it was left there.

What’s still untested: how the 2014 notebook split the text into documents (the justthese/ folder no longer exists), and running the complete 2014 software stack (R 3.1, tm 0.6, Rgraphviz 2.10) in Docker. That is the likely path to a perfect match, and the most work.

Reproducing

• Software used: R 4.6.1, tm 0.7.19, SnowballC 0.7.1, Rgraphviz 2.56.0 (bundled Graphviz 2.28.0).
• The matrices (*.RData) are not in git. Rebuild them with cap_week2/task2_corrplot-corpus.Rmd (current cleaning) and Rscript cap_week2/build_checkpoint_2014clean.R from the project root (2014 cleaning).
• Sweep the closest matches from cap_week2/:
  Rscript corplot_multigraph.R --checkpoint ../dtm_checkpoint_2014clean.RData --lf-min 8228 --lf-max 8228 --ct-min 0.870 --ct-max 0.9105 --ct-by 0.00001 --out ../out_2014clean
• Run sweeps with Rscript from a terminal, not the RStudio console, because they fork worker processes.
• WebP images are 800 px at quality 85. The *_wordpress.html files must be regenerated from the readable versions after any change.