In a scatter plot, I am graphing the results of taking 2 measures of the same part twice (column 1 is measure 1 and plotted as X, column 2 is the same part measured again, and is plotted as Y). I have added a trend line and include the equation of the line and the R squared value. Everything looks good, but what I do not know is:

1) How do I add a 45 degree line to a scatter plot in addition to the trendline?

2) How do I throw out the farthest Y value from the best fit line, and then graph a line parralel to the best fit line +/- that distance from the best fit line?

3) As I change the data entered into the chart, how do I force the scale to ALWAYS foce X and Y scales to be the same?

Thanks in advance for any help you can provide on this.