Need Some Simple Line Graph Help

I need to create some line graphs. The first one I need:
- A y-intercept of -1 (negative 1)
- A data point at (12, 11), 12 along the x axis and 11 along the y axis
- A line that connects from the y intercept to the data point and slightly beyond, possibly to a value of around 15.
The second one I need:
- A y intercept of 11
- Slope of -1
- A line decreasing from the y intercept of 11 until it reaches below the x-axis and stops at the point (12, -1).
- Then flattens out and continues straight for until (15,-1). Or however far.


Can anybody help me make these on excel? I would like the axis to count by intervals of .5 and I know this should be basic stuff but i'm blanking out right now.