Question

how to make a line graph of average monthly minimum and maximum temperatures

Answer 1

1
Draw a large cross in the middle of your graph paper. This represents the two axes y and x -- one vertical, one horizontal. The vertical axis is designated the Y-axis and the horizontal as the X-axis. The place where the lines cross is called the origin.
The areas below the X-axis and to the left of the Y-axis represent negative numbers. If your data set doesn't include negative numbers, you can omit those portions of the graph.

2
Label each axis with the variable it represents. To continue the temperature-time example from the introduction, you would label the x-axis as months during the year, and the y-axis as temperature.

3
Identify the range of data you have to include for each variable. To continue the temperature-time example, you'd select a range that was large enough to include the highest and lowest temperatures you plan to graph. If the range isn't very high, you can have a larger scale, spreading it out more so that it fills up the graph instead of just covering 10% of it.

4
Decide how many units every line on the graph represents for each of your variables. You might designate a scale of 10 degrees Fahrenheit (12.22 degrees Celsius) per line to measure temperature along the Y-axis, and a scale of one month per line to measure time along the X-axis.
Label several of the lines along each axis with the scale measurements. You don't need to label every line, but you should space the labeled line at regular intervals along the axis.