This code uses a ComboChart to emulate a ColumnChart, and applies annotations to a hidden line within the ComboChart which shares the same data as the columns. This code was inspired by a post by Dezső Róbert on the Google Visualization API forum: https://groups.google.com/d/msg/google-visualization-api/1yWwsXV-Ysk/PMJsxe7tzjgJ
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<div id="creativeCommons" style="text-align: center; width: 400px;"> <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_US"><img alt="Creative Commons License" style="border-width:0" src="http://i.creativecommons.org/l/by-nc-sa/3.0/88x31.png" /></a><br/><span xmlns:dct="http://purl.org/dc/terms/" href="http://purl.org/dc/dcmitype/InteractiveResource" property="dct:title" rel="dct:type">Code to add annotations to a ColumnChart</span> by <span xmlns:cc="http://creativecommons.org/ns#" property="cc:attributionName">Andrew Gallant</span> and inspired by Google Visualization API user Dezső Róbert is licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-sa/3.0/deed.en_US">Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License</a>.</div>