When using a server based or standalone versions of Ox Cal you can calibrate a single date from a simplified dialogue from the plotting utility (menu item @).This enables you to enter a lab identifier (or other label) the radiocarbon date (in The calibration curve can be selected.To view the curve you can also simply run an empty run file.If you include in the run file other calibration data-sets the curves for these will also be shown on the plot. In order to calibrate several radiocarbon dates at once click on the 'Projects...' button which opens up a dialog showing any existing projects.
Also you can press the [View curve] button from the opening dialogue (menu item @) of the plotting utility to generate a plot of the currently selected calibration curve.
Ox Cal is a very widely used web service for radiocarbon date calibration and analysis.
A package to hook into it from R would be very useful, I can't see any specifications for an API.
ratio might have varied over time, it was not until discrepancies began to accumulate between measured ages and known historical dates for artefacts that it became clear that a correction would need to be applied to radiocarbon ages to obtain calendar dates. As of 2014 this is the most recent version of the standard calibration curve.
There are separate graphs for the southern hemisphere and for calibration of marine data.
BCal is a related service, but it also doesn't have an obvious API.