How can I change the colorbar range to go from -20 to 20? [on mac computer, python 2.7]

n [31]: Part 3 import cartopy and netCDF4 ipylab inline impozt matplotiib.pyplot as plu import cartopy crs as cers import netCDFA from cartopy import config trom mpl toolkice. basemap import Basemap, from cartopy feature import NaturalBarthPeature. LAND, COASTLINE f rom cartopy mpl.gridiliner import LoKGTTUDE PORMATTER, LATITUDE PORKAPTER def temp height- (month) ric file2 # netCDF 4. Dataset( "hgt-mon. Itm.nc ".) lon = ncfile1.variables ["1on.)(92:121] Lat!= ncfile1 .variables [ "Zat.)( 14:27) temp nC filel.variables ['air, ] [month-1. 0, 14:27, 92:121] height = ncfile2 .variables ['hgt i] [month-1,15, 14:27. 92:121 Eig, axus states( ax #pit.axes (projection-cers. PlateCarree()) ax.gridlines () Piot filled contours, specitying projeotion used fox the input data (transform-...) plt contourf lon, lat temp, trans tormycors.Platecarree() pylab.colorbar (orientation-"horizontal) contr Pylab-contour (lon, lat, height, 4) Pylab.clabel (cont) re ( category='cultural, scale-'50n', name-adminlil states provinces shp ) states = NaturalEarthPeat facecolor-"none', lax add feature(states edgecolor grayh) Add coastlines ax.coastlines () pit.show() print temp. shape, height.shape temp height (0) Populating the interactive namespace from numpy and matplotlib