I really like your project! I've been wanting to do this for some time. I recently bought a Arduino compatible board but it has an onboard temp sensor; Lipo battery, solar panel input and microSD.
Atmel ATmega328P (8-bit MCU)
onboard 10-bit ADC
Maxim DS3231 I²C RTC, Digital Temp Sensor Output: ±3°C Accuracyhttp://www.seeedstudio.com/depot/seeeduino-stalker-v2-p-727.html?cPath=132_133
So far though I've just turned on the onboard LED with a GtkButton and in another project, retrieved the temp. and displayed it in a GtkEntry (one shot, not continuous)
I'm not sure how to advise you about the thread protection. It seams your requirement of 50Hz sample rate may be unattainable with GtkExtra. Here's a thread that talks about a solution for thread protection but they only needed up to 32Hz but maybe it can work for you. http://ubuntuforums.org/showthread.php?t=1463078
Your on Linux right? If so what version?
Here's a thread where someone was also grabbing temp and humidity from a PIC and plotting it but not in real time. Tadeboro gave some code which uses Cario directly. That might be what you need to get your required speed. Paul Childs mentioned in the same thread that he's "also developed a few plotting widgets customized for scientific publication quality graphing" I would PM him and or Tadeborohttp://www.gtkforums.com/viewtopic.php?t=3301#8764
I would also have a look at these:
"Goffice is a library including a 2D graph component with a cairo based backend. This library is used by Gnumeric and Gnucash."
"PLplot is a 2D and 3D plotting library with several cairo device drivers. Currently these include xwindows, ps, pdf and png."http://cairographics.org/examples/
Please post back or PM me. I would really like to see this work!