Scalability asks how the product behaves as load, data or users grow. A measure relates the property at increasing scale to the acceptable threshold. Scalability is a shape, not a point. The measure needs at least two load levels and a property that is expected to hold, such as response time or error rate. Linear growth, sub-linear growth and collapse are different results, and a single figure cannot tell them apart. The measure should state the axis, the range and the property, and it should be taken in an environment that resembles production closely enough for the shape to transfer.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.