Lines can be used to approximate a wide variety of functions; often a function can be described using many lines.

If a stock price goes from $10 to $12 from January 1st to January 31, and from $12 to $9 from February 1st to February 28th, is the price change from $10 to $9 a straight line?

How can I use two "pieces" of lines to describe the price movements from the beginning of January to the end of February?