changed $x_axis_value_interval_counter to -1

changed $x_axis_value_interval_counter from 0 to -1, it gives much better numbers

Before change ($x_axis_value_interval_counter = 0): 

$graph->setXValuesInterval(99); 
    output: 99, 199, 299...
$graph->setXValuesInterval(100); 
    output: 100, 201, 202...

After change ($x_axis_value_interval_counter = 1): 

$graph->setXValuesInterval(99); 
    output: 100, 200, 300...