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Unit 01 of 03

Chart Design

Measuring how much of a number reaches the reader as it passes through an encoding channel: which channel the comparison, distribution, composition, and relationship questions require, the shape that a histogram bin and a box summary cannot give back from a distribution, the read-back error that stacked and grouped displays produce on slices with no baseline, what a scatter and a heatmap carry from a relationship, separating trend from seasonality on the time axis, the area channel magnifying the ratio by its square, distortions beyond axis truncation, and the number of series that remain indistinguishable when color stands alone.

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Lessons

  1. 01 Chart Type Selection The same five regional means are passed through five encoding channels and read back, and the mean read-back error comes out as follows: position 0.070 m³, length 0.183 m³, angle 0.543 m³, color step 0.931 m³, area 2.468 m³. The 4.463 m³ difference between east and north is read back as 2.500 m³ from the area channel and 5.426 m³ from the angle channel, and not a single reading changes. The color step separates four of the five regions, with east and central falling on the same step. Each of the four question types asks for something different to be read back, and the channel pairing follows from that demand. 10 min read
  2. 02 Distribution Visualization The twin set that gives the same five numbers as the 921 readings of north is passed through three displays, and what is read back is not a number but a distributional feature: bin counts and the density display read back the two peaks of the twin, while the box summary carries no peak information at all. In the counts that pass through the length channel three bins drop to zero and 7 readings are erased from the display, and the 45.47 m³ extreme value of the twin disappears; the same extreme value is read back in north, because the tallest bar is 234, not 329. When the bin width is 12 m³ the number of peaks read back falls from two to one, and when the kernel width is 0.5 m³ four peaks that are not in north appear. 14 min read
  3. 03 Categorical Visualization The crosstab of five regions and four tariff tiers is encoded with two displays, and the difference shows up at the baseline: in the stacked display the bottom slice is read from the position channel and its relative error is 2.7%, while the upper slices are read from the length channel and the error is 22.0% in the third slice and 47.2% in the fourth. North's fourth slice of 8 readings and west's third slice of 11 readings are read back as zero, that is, they leave the display entirely. Of the 39 region pairs that must be separated, the stacked display separates 35 and the grouped display 39; no pair is read reversed, so what is lost is not the order but the distinction. The same stacked display gives the total with a single reading: 921 observations come back as 920. 13 min read
  4. 04 Relationship Visualization When 1,101 subscriber pairs are encoded as a scatter plot, the mark width leaves 1,600 distinguishable cells, and 792 of the points, that is 71.9%, overlap and can no longer be read back; the densest cell holds 16 points. When the same pairs are put on a heat map, not a single point is lost but the count is: the 28 distinct count values in the source fall to six of the seven color steps, 37 of the 51 filled cells land on a single step, and the mean relative read-back error comes out at 280.9%. When the steps are divided into an equal number of cells instead of equal intervals, the same error falls to 15.7%. A line that gives a single y for each x reduces the cloud's mean span of 44.1 steps to a single step. 11 min read
  5. 05 Time Series Visualization The same 36-period series is encoded at the same height with two axis scales: on the axis that starts from zero the reading resolution is 2.273 m³ and the mean read-back error per point is 0.540 m³, on the truncated axis 0.909 m³ and 0.246 m³. At a precision of 0.05 m³/period the slope is read back only from a window of 24 periods on the axis that starts from zero and from a window of 16 periods on the truncated axis. The seasonal component has a slope of its own and changes sign with the start of the window: the same 24-period length gives -0.2239, -0.0487 and -0.2260 m³/period in three separate places, while the trend put into the fictional series is -0.140. 6 min read
  6. 06 Geographic Visualization Five regions are encoded on a 12x12 coverage grid with five color steps, and what the reader reads back is not the step but the area the step covers: the correct mean weighted by the number of readings is 19.713 m³, while the read-back weighted by area gives 18.484 m³, a difference of -1.228 m³. West carries 14.9% of the readings and 25% of the area, so it gets a weight factor of 1.680. Projection stretching suppresses two regions of equal area by a factor of 1.237 relative to each other. When the area share is made equal to the read share, the error falls to 0.396 m³, at the price of 93 cells changing region and the pairs of neighboring regions dropping from 8 to 4. 14 min read
  7. 07 Misleading Visuals The true ratio of 1.2208 between east and north gives a bar ratio of 3.0769 when the axis is started from 18 m³, and a reader who takes length as proportional to value reads north as 8.019 m³ instead of 20.210. When the same ratio is encoded into a square, both sides are multiplied, so the area ratio comes out as 1.4905 and east is read as 30.123 m³. For two series with different units, nine defensible axis pairs give seven separate crossing periods. When 12 irregularly spaced periods are laid out at equal intervals, a slope of -0.2079 m³/period is read as -0.6635. All four arise from a defensible default behavior. 9 min read
  8. 08 Accessible Charts When eight series are encoded with a default palette made of two hue families and four lightness steps, all eight are distinguished by the full decoder, while a decoder that does not read the hue distinction is left with only four. When the shape channel is distributed in sequence, the number stays at four, because the shapes start over after four series and the color collision also occurs four series apart; when the shape is distributed by collision, it rises to eight. A legend box asks for 16 transitions, an ordered legend 6, direct labeling 0. Of nine lightness steps, five are distinguished at an interval of two, and with four shapes the ceiling of a single display comes out as 20 series. 14 min read

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