[R] superscript characters in title with '+'

[DrCJones]

Hi,

How can I get the '2+' into superscript in the following title: 

'[Ca2+]i onsets' 

I tried the command below, but it doesn't work. What am I missing? 

hist(X, main=expression(([Ca*]i^2+) 'onsets'),  xlab = 'sec')




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