It's reasonable to assume many people will build sparklines once and
cache them, using the php command line utility like so:
php deficit.php > deficit.png
If this execution fails (perhaps because it can't find an existing log
file), the person executing the command doesn't see an error,
because it's been routed into what she presumes to be an image file.
The warning should be written on stderr, where she will see it, not to
stdout, where it'll more than likely get mixed up with image data.
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