Instantaneous velocity in a graph

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Homework Statement

http://homework.sdmesa.edu/cmona/195/MP195AFiles/1%20linear%20kinematics%20lab.pdf
That is the link to my lab.
My question is on page 5:
Plot instantaneous velocity as a function of time.
Recall that your data only indicates the average velocity of the glider for each time interval and that
for constant acceleration, the instantaneous velocity of the glider at the middle of each time interval is
equal to the average velocity of the glider during that time interval
Plot the instantaneous velocities of the glider as a function of time. Remember that your first velocity
occurs at 0.05s, and the next is at 0.15s, 0.25s, …etc.

My data:
X(CM)-T(S)
0-0
4.9 - .1
10.8 - .2
17.5 - .3
25.2 - .4
33.8 - .5
43.3 - .6
54.1 - .7
65.1 - .8
77.3 - .9
90.2 -1.0
104.2 - 1.1
119.2 - 1.2
134.9 - 1.3
151.7 - 1.4

Homework Equations

I have no idea how to graph the instantaneous velocity, I tried and tried, but for some reason the graph i get is a curve, a very steep curve, i was told it should be more of a line. This curve also screws up later calculation in the same lab.

The Attempt at a Solution

I tried following the instruction, from what I understand, using my first graph points. I take the .15s and trace it and see what velocity I get and use that as my instantaneous velocity... I know that is wrong because every time I try that my numbers are way off!

 

[AvroArrow]

I was told it should look like a line graph, but when I try that, I end up with a curve. Please please help!