Answer:
The well-desgined graph has an appropriate scale for the y-axis. By appropriate we mean that the scale is such that all points on the graph can be discerned easily. In other words, the scale is not too wide or too narrow.
For graph A, the scale is such that the data points are at such a distance from each other that their values can be easily read off. On the other hand, for graph B, there isn't enough vertical distance between the data points for them to be easily read.
