1. Proper Plots/Tables: Part 3 in this link.
2. Significant Figures: Part 2 in this link.
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1. The Uncertainty of Analog Power Supply.
Typically, the uncertainty associated with a measurement that is read off of a scale – with small distances between the markings – is estimated to be half the size of the least count.
a. In the case of the analog voltmeter on the DC power supply, the scale is marked in 0.5 volt increments. The uncertainty is therefore 0.25 volts. However since you have only 1 decimal digit, 0.3 volts is the rounded uncertainty.
b. The uncertainty is constant and does not depend on the measured voltage.
2. Error of DMM.
The error should be calculated using the equations in the manual. The number of decimal places (ie., for 3.14, the number is 2; while for 31.412, it is 3.) should be the same for the measured data and its error.
3. Significant Figures for LINEST() results.
Your data has at most 4 digits, so does your fitting results.
4. Plot of Error Bars.
Do not forget the error bars for your plots. If they're too small to be seen, you should point that out in the figure legend.
5. Units of Slope and Intercept.
Slope should have units. However, in this case, does not have a unit. But the intercept does have unit of V (Volt).