.py codes:

 @api.multi
 def plotfig(self,cr):
   x=[1,2,3,4]
   y=[4,7,9,8]
   plt.plot(x,y)
   tem='/tmp/%s.png' % cr['uid']
   plt.savefig(tem)
   pic_data=open(tem,'rb').read()
   self.write({'Pic':base64.encodestring(pic_data)})
   os.remove(tem)

Now when user create a record and clicks button "plotfig", a figure will be shown on "Pic"; all seams ok untill now;

but if the user create another record and clicks button "plotfig" again, he/she will get warning: "RuntimeError: main thread is not in main loop"; sometime the warning is "Fatal Python error: GC object already tracked Aborted" / "Segmentation fault" and the Odoo server will shut down automaticly.

if I click "Ctrl+c" to stop Odoo server, I will also get warning: "RuntimeError: main thread is not in main loop";

I don;t know how to resolve these problems.