Colors - I wanted to use opposite colors for e-mails in and e-mails out so I chose purple and green because of their placement on the color wheel. I wanted Promotion, Social and Primary e-mails to be a similar hue because they were in the same category of incoming messages.
Graph Orientation - Initially I gravitated towards the code from the colors and contrast codepen example because I understood it, but that was because it didn't leverage functions. As you can see from part I, my first pass at a D3 graph had so much hard coding that I couldn't make it interactive so I overhauled the code. There were way fewer examples of column charts on the web and this reinforced my decision to change the graph orientation. I feel it is much easier to focus on the y-axis values when analyzing this dataset.
Chart Type - The data is focused on the classification of my e-mails which is a categorical variable. Tufte's legacy demonstrates the efficacy of a simple bar chart so I chose that for my graph type. The rectangle shapes are easiest for humans the understand and interpret the meaning of the data.
Legend Filtering- While a label next to the series would have reduced cognitive load for the users, I wanted to give the users the ability to filter the series shown on the graph and the clickable legend accomplishes that magnificently. This feature of D3 really excited me because I'd never seen anything as slick in excel or ggplot2. The interactive hover functionality makes it a little easier to understand what each color represents; mitigating my initial concerns.
Average Line - Facilitates quickly deriving summary statements about the messaging categories, especially when combined with the percentage of the total switching feature. I found this feature very challenging to create and integrate with the legend filtering feature, but it really pushed my understanding of how to work with D3 and more specifically how to manipulate data with javascript.
While I did a ton of research to get me through debugging and I also referenced many of the in class/OH materials, I want to call out the resources below as they were the building blocks that guided my newbie tinkering. As with any coding project, building off of the online repository exposed me to far better coding practices and a resulted in a more impressive visualization/analysis.
https://bl.ocks.org/mbostock/3887051 - Baseline
https://bl.ocks.org/Andrew-Reid/64a6c1892d1893009d2b99b8abee75a7 - Grouped Bar chart filtering
http://bl.ocks.org/kaz-a/58224255b41e64810652d89b8f976673 - Average line
https://bl.ocks.org/mbostock/3885705 - Formatting pct.