You can switch to the metric system on their website [1]. The graph can also be switched to metric by changing the "uom" query parameter from "E" to "M"
It would be better if NOAA used kilopascal (an actual SI unit) instead of millibar, but I guess the conversion is not too bad since 1000 mbar = 100 kPa.
The NOAA data isn't consumed solely by scientists though. I'm sure millibars were chosen as the metric measurement of pressure because people actually use them, such as mariners.
[1] http://www.ndbc.noaa.gov/station_page.php?station=42058&unit...