Line charts are used to compare changes in data for a time-limited period. For example, you might want to compare email open or click-through rates for two consecutive quarters, compare 3rd quarter 2017 with 3rd quarter 2016, or compare 2016's release cycle email open and click-through rates with 2017's.
In the preceding image, you can see two sets of data, in this case open rates from 2015-2016 and 2016-2017. The line chart gives a good visual comparison, but it doesn't provide much context. Limiting the data sets to specific periods or types, for example release emails, will give the data better context.
A bar chart is particularly useful for comparing changes between data sets, especially larger changes in similar data sets. A comparison of KPI totals for three or four years, or of KPI totals for quarterly campaigns, is well-suited to bar chart display.
The image above shows a comparison of KPI data by quarter. In this case, the data sets are grouped by quarter. You could also group by KPI data set and add industry benchmarks. Bar charts are
Use a Pie Chart when you want to visualize data as part of a whole. Select a metric, like your email marketing unsubscribe rate, and figure out what type of email generates the most unsubscribes? Are you sending too many sales or promotional emails? Or do you need to refresh your newsletter?
In the above image, the chart shows unsubscribes by type of email sent. While this type of comparison is helpful to see what type of email generates the most opens, clicks, or unsubscribes, it doesn't give the data context. Yes, most of the quarterly unsubscribes came from Newsletters, but more of those were also sent over the quarter.