Right,
TFuncSeries draws a function which is defined by the OnCalculate event: whenever the series needs a value it fires this event, passes the x value of the point to be drawn and you return the corresponding y value. I once wrote a tutorial for it:
https://wiki.lazarus.freepascal.org/TAChart_Tutorial:_Function_SeriesTExpressionSeries goes a step further. Here the function is defined by a string, like your "m*x+b" (you must drop the "y ="), and it employs the FPExpressionParser to evaluate this formula (
https://wiki.lazarus.freepascal.org/How_To_Use_TFPExpressionParser).
The symbol "x" is pre-defined to identify the variable to be plotted along the x axis.
"m" and "b" are user-defined parameters. After adding the series to the chart, select it in the object inspector, go to property
Params. Click the "..." button and add two parameters. Select the first parameter and define its
Name to be "m" and set the
Value to what you need. Then select the other parameter and repeat with "b". Type the formula "m*x+c" in the property
Expression. The chart draws the curve, but this is not yet very satisfying because you also must tell the x range: go to the series'
Extent and set
XMin and
XMax as needed, and activate the range by setting
UseXMin and
UseXMax to true. And you should also set
ExtentAutoY to true so that the y data range is found automatically (otherwise you must do the same with the y Extent). Now the series should be plotted correctly without writing any line of code.