Since base is narrow and collector is heavily doped with respect to the base, current can flow in the collector circuit though collector-base diode is reverse bias. The amount of collector current depends on how well base-emitter junction is forward bias. Thus large the forward bias VBE and the base current IB, larger will be Ic for a given Vcc.
Hence output or collector characteristics curve is drawn for Ic—->VcE for different values of IB
(Click on the following image for more explanation)