No class 18 – exam last Thursday. And next Thursday is Veteran’s day, so it’s hard to establish any continuity these weeks. And Thanksgiving is coming right up.

Today’s class started with remembering the electricity bill computation we started last week. I developed the idea of a function in parallel with this example and its generalizations – dependent and independent variables, representing a function with a table or a graph, only sometimes with a formula.

Graphing a function in Excel calls for a scatter plot, not the line chart you might expect. We worked with the electricity bill example from the text.

I did discover a strange Excel feature/bug. Suppose you select two columns of numerical data and build a line chart (by mistake). Excel thinks each row is a category for which you have two pieces of data. It labels the categories 1, 2, … (row number) and shows a line for each. Then you realize that what you wanted was an x-y scatter. So you change the chart type – and get two scatters, one for each column. The only change is that the numbers 1,2, … that used to label the categories now label the points on the x axis. That really surprised me. The only way to get what you want is to choose the right type (scatter) from the start.

Sorting that out on the fly to my satisfaction took a fair chunk of class time. I think only a few of the students followed what was going on.

Once I had the correct scatter plot we did some what-if analysis – changing values of the independent variable, adding rows, changing slope and intercept.

The names “slope” and “intercept” came last, along with the memorized “y=mx+b”.

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