Scatter pictures with Google Charts

2008-04-25, , , , Comments

In a recent post on his blog Matt Cutts asks:

I almost wanted to call this post “Stupid Google Tricks” :-) What fun diagrams can you imagine making with the Google Charts Service?

Here’s a stupid trick: you can use the Python Imaging Library to convert a picture into a URL which Google charts will render as the original picture.

Here’s the original picture:

Spider

here’s the version served up by Google charts:

Google Chart Spider

here’s the code:

import Image
import string

def scatter_pixels(img_file):
    """Return the URL of a scatter plot of the supplied image
    
    The image will be rendered square and black on white. Adapt the
    code if you want something else.
    """
    # Use simple chart encoding. To make things really simple
    # use a square image where each X or Y position corresponds
    # to a single encode value.
    simple = string.uppercase + string.lowercase + string.digits
    rsimple = simple[::-1] # Google charts Y reverses PIL Y
    w = len(simple)
    W = w * 3
    img = Image.open(img_file).resize((w, w)).convert("1")
    pels = img.load()
    black_pels = [(x, y) for x in range(w) for y in range(w)
                  if pels[x, y] == 0]
    xs = "".join(simple[x] for x, _ in black_pels)
    ys = "".join(rsimple[y] for _, y in black_pels)
    sqside = 3.0
    return (
        "http://chart.apis.google.com/chart?"
        "cht=s&"                          # Draw a scatter graph
        "chd=s:%(xs)s,%(ys)s&"            # using simple encoding and
        "chm=s,000000,1,2.0,%(sqside)r,0&"# square black markers
        "chs=%(W)rx%(W)r"                 # at this size.
        ) % locals()

and here’s the url it generates:

http://chart.apis.google.com/chart?cht=s&chd=s:DDEEEEEEE…&chs=186x186


Smallprint. Google charts may return a 400 error for an image with a long URL (meaning lots of black pixels in this case). The upper limit on URL length doesn’t seem to be documented but a quick trawl through topics on the google charts group suggests others have bumped into it too. Connoisseurs of whacky pictures should pay CSS Homer Simpson a visit.