Do you have a create a scatter plot to write a linear model?
It depends, to be honest.
It depends mainly on the type of data you have. If you have exact data (with a very lucid correlation between the
When you have real life data (data from science experiments, statistics, etc), then you usually don't have a perfect correlation, simply due to the nature of humanity. When documenting this type of data, it is handy to set up a scatterplot of your data, and then create a trend line (or best fit line) - which is a line that passes close to all the points on your scatterplot (but doesn't necessarily touch any of them). You can then write the equation of this line, and it will be your model.
Here is a video that explains this, just in case:
Hope that helped :)