Varying emitter base voltage of a BJT in LTspice
For the x-axis, select the waveform window, hover the mouse over the X-axis until the cursor changes into a ruler, then RClick on it and change time to V(Ve,Vb), or V(Ve)-V(Vb) (same thing). It may not work since you seem to have assigned two labels to the same node. LTspice will take precedence to the last assigned name (except for the ground).
For changing the BE voltage, you'll have to either change the parameters of the transistor .model -- I doubt this is what's needed -- or change the resistor network such that the DC operating point changes. Since this is an assignment, I'll leave that up to you. Is C1 meant to be tied to ground, or was there supposed to be a load afterwards? I ask because your source is modelled as a sine.