Since only data you have is this table, you don't need to connect the points and speculate on if its graph is linear or not.You can't really know its properties with this much information. Each interval can either be linear or nonlinear on its own. Therefore, you can just leave it like this:
If you really want to sketch the velocity-time graph(again you don't have to with just this much information), you can find the average velocity for each interval and sketch it accordingly. However, take these points into consideration while sketching it:
- What you essentially need to accomplish is to make the area under the graph at each interval to be equal to $\Delta x$ of each interval.
- The average velocity for each interval would be $2.2$ $m/s$, $-1.4$ $m/s$, $-3.8$ $m/s$, $-3.7$ $m/s$, $-1.6$ $m/s$ respectively. However, you can't really set these values in the graph since you can't know at what point these velocities are reached. You need to know instantaneous velocity of the object at each time to accurately sketch the graph.(which will require $x(t)$ function)
- The graph will just be an approximation and probably not even a close one.