Beginning Python - From Novice to Professional

CHAPTER 21 ■ PROJECT 2: PAINTING A PRETTY PICTURE 415
Constructing Some PolyLines
To create a line diagram (a graph) of the sunspot data, you have to create some lines. In fact,
you have to create several lines that are linked. ReportLab has a special class for this: PolyLine.
A PolyLine is created with a list of coordinates as its first argument. This list is of the form
[(x0, y0), (x1, y1), ...], with each pair of x and y coordinates making one point on the
PolyLine. See Figure 21-2 for a simple PolyLine.
Figure 21-2. PolyLine([(0, 0), (10, 0), (10, 10), (0, 10)])
To make a line diagram, one polyline must be created for each column in the data set.
Each point in these polylines will consist of a time (constructed from the year and month) and
a value (which is the number of sunspots taken from the relevant column). To get one of the
columns (the values), list comprehensions can be useful:
pred = [row[2] for row in data]
Here pred (for “predicted”) will be a list of all the values in the third column of the data.
You can use a similar strategy for the other columns. (The time for each row would have to be
calculated from both the year and month: for example, year + month/12.)
Once you have the values and the time stamps, you can add your polylines to the drawing
like this:
drawing.add(PolyLine(zip(times, pred), strokeColor=colors.blue))
It isn’t necessary to set the stroke color, of course, but it makes it easier to tell the lines
apart. (Note how zip is used to combine the times and values into a list of tuples.)