Currently davaxi/sparkline is set to the version dev-multiple-series in composer.json

This results in the following error when running composer require piwik/piwik

Problem 1
    - piwik/piwik 3.13.1 requires davaxi/sparkline dev-multiple-series -> no matching package found.

I believe davaxi/sparkline should instead be set to ~1.2, to install the latest release that includes the changes merged to support displaying multiple series.