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       style="border:1 solid black;" alt="Rainfall amounts"></iframe>

Changing the border style should have the effect you want.

=head3 How do I add a grid under the sparkline?

The L<SVG::Sparkline> does not currently support grids. I am concerned that
adding a grid might also violate Tufte's intent for the sparkline.

At present, the best solution would be to generate a grid of the appropriate
dimensions and overlay it with the sparkline. This only works if the sparkline
has no background color, which leaves the background transparent.

=head1 AUTHOR

G. Wade Johnson  C<< <gwadej@cpan.org> >>

=head1 LICENCE AND COPYRIGHT

Copyright (c) 2015, G. Wade Johnson C<< <gwadej@cpan.org> >>. All rights reserved.