Dr. Hiroshi Ishikawa（石川宏 博士）has a geiger counter in Hino that is posting realtime radiation readings. He is also posting raw data to an archive page (so far through 3/18). I graphed and posted those data below. I'll update this if/when Dr. Ishikawa posts more raw data.
From Dr. Ishikawa's English FAQ (translated by Alex Boyd), here is information for converting to sieverts.
Thank you for providing continuous data on radiation levels. I am much relieved by the data you publish.I've been checking this page often, and since the two spikes, I have seen no other excursions. To put this into perspective, even the spikes were not dangerous.
I am not an expert, but I do work with radiation, so I researched the CPM / Sv/h conversion.
I expect the biggest issue now is radioactive Iodine 131, which is created by Uranium fission. Iodine 131 produces gamma radiation principally with energy of 365 keV, and according to the graph on the manufacturer's homepage this gives a relative countrate of approximately 1. Therefore 1 CPM ~ 1 mR/h = 0.01 uSv/h is appropriate.
It isn't clear what the relative countrate is relative to, but the values given for Cesium 137 (Gamma at 662 keV) and Cobalt 60 (Gamma at 1.17 MeV / 1.33MeV) are roughly the same so there should be no problem.