Database Reference
In-Depth Information
Figure 9-4. Strikeout line chart with trend line added
Researching the rules of the game, we discover that the mound was lowered and the strike
zone was reduced in 1969. The league implemented these changes in response to the increas-
ing dominance of pitchers in the league. They wanted teams to score more runs during
games, in hopes of boosting attendance. We can see that this change to the rules coincides
with a decrease in the average number of strikeouts per game that lasted until 1981, when the
trend once again began climbing.
We can add a vertical reference line at 1969 by right-clicking in the chart area and selecting
Add Reference Line
, and then filling out the resulting dialog box as shown in
Figure 9-5
, res-
ulting in the updated line chart shown.
So far, we've only been able to show how one quantitative variable has changed over time.
While we've added a trend line and a reference line, we haven't yet matched the degree of
sophistication of Playfair's original line charts.
Let's see how we can gain further insight into the history of strikeouts by adding a second
line to the chart: pitchers per team.
