At the end of a ggplot, this works fine:

+ opts(title = expression("Chart chart_title..."))

But this does not:

chart_title = "foo"
+ opts(title = expression(chart_title))

nor this: 

chart_title = "foo"
+ opts(title = chart_title)

How can I add a title to a ggplot when the title is a variable name?

r ggplot2 title
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