On previous installations I showed you how to get sales rank
information about a book from Amazon, and how to create a graph of the
data over time. Now my vengeance will be complete: I will unleash
sparklines upon the world!
Sparklines are really interesting: bits of anonymous data that add
quantitative analysis to text, in some cases without breaking the flow
of a sentence. The meaning of a sparkline depends on context or a tiny
textual label, not on big sets of axial markings. I've been a fan of
sparklines for a while, and what better
way to propagandize them than by inclusion into a book of cool tricks?
THERE IS NONE. So here we go with the final part of the trilogy:
As you know, Bob, in previous episodes we defined a
Like Gruff, the Those images from the Crummy homepage (Cookbook A sparkline needs to be small, and unlike Gruff,
This is fine. I don't need the whole history for a sparkline like I
do with a big graph. I'll settle for the sales rank history from the
past 30 days. But I run my data collector every hour, and that's still
24*30=720 data points. Solution: I go back 720 hours, and skip ahead
24 samples at at time. This way I pick a sample sales rank for every
day, ending with the most recent sample. You can change
Now I have an array and I just have to send it to be turned into a
sparkline:
Well, not quite. Earlier I mentioned that In this case, our numbers are already between 0 and 100, since
we're taking the reciprocals of numbers greater than one. But they're
also between 0 and 1, as we saw in the previous installment. When the
range is 0-100, this makes for boring sparklines. If your book
rocketed to the top of the Amazon charts and stayed there, its amazing
success would look like this:
To make our data visible, we need to stretch it out. We might as
well go for maximum stretch: treat the smallest sampled rank as zero,
and the largest as 100. That's what the I gave a similar method in the sparklines recipe in the book, but
it's hard-coded to scale a range to 0-100. This implementation is more
general and you can use it anywhere. I really should have made this a
separate recipe because it kept coming up. In 12.14 "Generating MIDI
Music" (which I hope to write about later) I reused this formula for a
different range and got it wrong, and had to send in an
erratum. In 2.12 "Using Complex Numbers" I used a similar formula to
scale an ASCII drawing of the Mandelbrot set to any desired size. So
it turns out this is a useful method to have.
Anyway, now we can scale any data set to take maximum advantage of
the vertical space allotted us by a sparkline. In the book I lightly
discuss the ramifications of scaling all these different data sets to
the same 0-100 range. The Cookbook sells much better than Beginning
Python, but you can't tell that from the sparklines. Which is fine
here, because I want these sparklines to show trends.
The The full sales rank monitor program is available here. I hope other authors
find it useful.
What's next for Best of Ye Book of Ruby-Receipts? I don't
know. These promotional tutorials take surprisingly long to write:
it's almost as much work as was writing the original recipes. So I may
give it a rest for a while. But let me know if there's a recipe in the
Cookbook you'd like to see me explore in more detail, in the context
of a real-world program like the sales rank monitor.
![[Comments]](/nb//themes/Default/comments.png "2 comments")
(2) Sun Jul 30 2006 19:42 Anthology Of Ruby Cookbook Recipes That's Not The Ruby Cookbook Itself #3: Sparklines:
Okay, thanks to much work by Kevin the nameserver is back up, and I can return you to the Book Sales Trilogy, where I sell the Ruby
Cookbook using software that tracks how many copies people have
already bought. That's what I call recycling!
SalesReport class which encapsulates information about a
product's sales rank over time. Then we extended the class to give it
the ability to write out a big graph describing the history of its
sales rank. Now we're going to extend it again, and give it the
ability to write out a sparkline.
sparklines gem is a great piece of
code by Geoffrey Grosenbach. It's simpler than Gruff because
sparklines are simpler than graphs. It does have some issues you need
to watch out for, though. I cover sparklines in Recipe
12.5, "Adding Graphical Context with Sparklines". In the book I give
some silly examples focused on embedding sparklines into HTML pages
with the data: URI scheme, and incorporating sparklines
into Rails views with the sparkline_generator gem. Here,
I'll show you to show how to write sparkline graphics to static PNG
files.
and
Beginning Python 
) show
sales rank values for the past 30 days. How do I make them? Let's see.
class SalesReport
# Make a sparkline for the sales of this product.
def make_sparkline(graph_path, time_units=30, samples_per_unit=24)
path = File.join(graph_path, "#{@asin}-salesrank-sparkline.png")
# Gather a sample of the data
sample = []
(size-(time_units*samples_per_unit)-1).step(size-1, samples_per_unit) do |i|
sample << 1/self[i][1]
end
sparklines doesn't compress data points. Gruff takes an
image size in pixels, and whether you give it 8 data points or 800,
your graph is that size. But if you give sparklines 800
data points, you get a really long sparkline. I can't use all the data
I've gathered since these books showed up on Amazon: I need to
economize.
time_units and samples_per_unit to customize
your sparkline.
Sparklines.plot_to_file(path, scale(sample), :type => 'smooth',
:line_color => 'black')
end
end
sparklines
line graphs don't compress data horizontally. There's a fixed number
of pixels between each point. Well, sparklines doesn't
compress data vertically either. A line graph can handle values
between 0 and 100. You can make the sparkline bigger but I'm pretty
sure you can't increase that range. If you give values outside that
range they get clipped or ignored.
(caution: embedded
sparkline not visible in IE).
scale method
mentioned above does. Here's the definition:
# Scale data so that the smallest item becomes 0 and the largest becomes
# 100.
def scale(data, bottom=0, top=100)
min, max = data.min, data.max
scale_ratio = (top-bottom)/(max-min)
data.collect { |x| bottom + (x-min) * scale_ratio}
end
sparklines gem lets you do other kinds of
sparklines: pie charts are my favorite, as you'll see in the book. I'm
really excited about the possibility of sparklines: they're like the
little crawls at the bottom of television news stations, except
they're classy and related to the main text.
- Comments:
Posted by Aaron Swartz at Sun Jul 30 2006 20:44
"show pagerank values for the past 30 days"I think you mean salesrank
Posted by Leonard at Sun Jul 30 2006 20:48
Fixed, thanks.
