What did the $I_C$ vs $V_{CE}$ chart tell us about changing input current?

This plot has different curves corresponding to different values of $I_B$, the current into the base. Each curve has a roughly-constant-$I_C$ plateau corresponding to the ``linear active region'' where the transistor is biased and has a $V_{CE}$ above a certain value. In this region, the plateau $I_C$ value is linearly dependent on $I_B$. So we can control what $I_C$ is by choosing $I_B$.