You could use the Desmos Graphing Calculator to explore straight lines.
Suppose you want to have a look at the graph of y = 2x+1. All you need do is click and type your function.
You can use sliders in the Desmos calculator to explore families of graphs. If you type in y = mx +c you will be given the option to add sliders. Select all to add both sliders:
Now click on the image below and try changing the values of m and c and see the changes to the graph.
With m=2 and c=1 we can see the graph of y = 2x+1 which has a gradient of 2 (note that for every one unit across you go two units up) and an intercept (where the line crosses the y axis) of 1.