try this:
1) Copy all of the example code I posted from <html> to </html>
2) Open notepad.exe
3) paste the code in
4) save the file on your computer as test.html
5) open test.html in a web browser
If that works, then it shows you how to set up your app.
If you are not familiar with HTML and JavaScript, you may want to learn a bit about those.
If you are familiar with MATLAB you may use the MATLAB analysis app to calculate the percentage of dew point and use the thingSpeakWrite function to write it to a new channel or field. Then modify the example google gauge plugin to read the value from that field and display it.