The Oregon WMR-100 and WMR-200 have very low barometer accuracy specifications. It is a fault of the barometric sensor in those models. Nothing is broken. They are working according to design specifications. It doesn't detect changes down to the level of precision that you are looking for. Every WMR-100 and WMR-200 station who's data I look at suffers from the same design issue. The results are that you end up with very jagged graphs that go up and down abruptly by a few millibars or as you noted at least 0.2 inHg.
You can see this right on the specifications stated directly from Oregon Scientific:
https://www.oregonscientificstore.com/p-31-oregon-scientific-wmr200a-professional-weather-station-center.aspx+- 10 mb = +- 0.3 inHg. That is a lot of variation and extremely low resolution.
It is no wonder you would have a hard time calibrating such a station. It is impossible for the station to ever seem to be accurate (maybe it is spot on 10% of the time). The best you could hope for with this type of hardware is that your average could be a slim resemblance of the actual barometric pressure and you'd be able track larger trends spanning days but not so well for daily fine details.
For example just look at this Oregon Scientific station and how its blue line that has low precision tracks compared to the red line which is more precise and representative of the actual pressure.
https://weather.gladstonefamily.net/site/D1843(not picking on this station if this is your station...this is just how all WMR-100 and WMR-200 stations work)
As you can see the blue line does not track perfectly with the analysis red line with any OS hardware of that generation (not certain about newer OS stuff). But it is still able to pass MADIS quality checks, because the average and general tracking follows the given trend.
You can compare that to my station that has better barometric sensor yielding beter accuracy and precision, which when properly calibrated it looks like this.
https://weather.gladstonefamily.net/site/F3708You can see the difference in the blue barometric line that a station with a better barometric sensor produces. There is more detail and smaller increments.
Your Oregon Scientific stations are indicative of the types of more affordable stations that were produced quite a few years ago. Newer model stations have better sensors, even new stations that should meet your budget are incredible in comparison. But do your homework before picking a replacement as there are still new stations being sold that don't cut the mustard. There is nothing you can do other than to seek a replacement for a different newer model station.
So there is nothing wrong with WeeWx. It has to do with the quality of data you are feeding it based on the design limitations of your barometric sensor. In the OS example I used the station was using Weather-Display software. And I could have selected a Meteobridge Oregon Scientific station and you would have seen the same poor results. You can look at countless other examples of other station brands/models running off WeeWx, Weather-Display, Meteobridge (like mine) and you would notice great results with higher precision if the hardware sensors are better.
Here is a good station list to work off. Every time you select a WMR-100 or WMR-200 off this list you will see similar results no matter the software used. Then look at some of the other hardware station brands/models and see which produce nice results. Keep in mind that some stations have broken sensors...so don't look at just one example for each type to make a determination.
https://weather.gladstonefamily.net/cgi-bin/wxequip.pl
