Just tested a simple timechart in Splunk 7.2 and indeed, when locale is set to en-US, it shows 12h format in the graph x-axis (and also in the event view), but it is indeed showing in 24h format in the table below the graph (and in the Statistics view).
Anyway: if you want it to be 24h everywhere, just ensure your locale is set to en-GB, either by changing browser settings, or by changing the url in the address bar.
Might be worth checking with Splunk Support if this is by design, or a bug.
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