How do I best visualize this voltage data for a science project?
The 7 steps in conducting a scientific experiment are:-

1) Pose a Testable Question: 

2) Conduct Background Research: 

3) State your Hypothesis: 

4) Design Experiment. 

5) Perform the Experiment.

6) Collect Data.

7) Draw Conclusions.

Now you are at step 7, and you have a quandary. The data doesn't appear to support your hypothesis! But hold on a minute - exactly what were you expecting, and why?

Your research should have told you how and why wire length might affect the charge voltage, and how to design an experiment which would give meaningful results. So let's examine the experiment to determine what results you could expect from it. Then you can organize the data to see if it supports the theory.

The factors that might have affected the end of charge voltage are:


Light intensity.
Charging time.
Wire length.
battery voltage before charging.


Battery voltage increases roughly proportionally to the amount of charge put into it, and the solar panel is capable of putting out a current proportional to light intensity. But the resistance of the wire might restrict charging current to a lower value, which should result in lower battery voltage. Wire resistance is proportional to length. You are hoping the increased resistance of the longer wire will have a measurable effect. 

So you could plot a graph with wire length on the X axis and battery voltage on the Y axis. But to get meaningful results you need to separate out the confounding factors of light intensity, charging time, and battery starting voltage. 

For voltage you can subtract start voltage from end voltage and show the difference. 

To remove charging time from the result, divide the voltage increase by time to get eg. volts per minute. To account for light intensity you could plot 3 curves, one for each intensity (mildly cloudy, clear sky, sunny).

If the graph shows a clear relationship between wire length and voltage increase for all three light intensities then the results support your theory. A stronger effect at higher light intensity might indicate that the panel is restricting current in lower light. If results appear to be 'random' then perhaps the wire has too little effect, or some measurements are not sufficiently precise (light intensity) or not a good proxy for charging current (battery voltage). 

Ohm's Law says that Resistance = Voltage / Current. The higher the resistance the less current will flow in a circuit, so a longer wire should reduce current and increase the length of time needed to charge the battery. But the solar panel and battery also have resistance. If that is much higher than the wire resistance then wire length will have little effect.   

If the experiment does not show a 'positive' result it doesn't prove that the wire length has no effect, but just that it is not significant in this case. This is good news, because it shows that the panel can be positioned a considerable distance away from the battery without adversely affecting charging time. Therefore if this is the result then the experiment was a success!