void rtlsdr_skyline::set_chart_grid()
{
	if (display_in_calib_mode)
		return; // no modification needed

	if ((from_freq_rounded == 0) || (to_freq_rounded == 0))
		return;  // it's too early
	
	// grid spacing along axis X
	double x_tick = chart1_optimal_major_unit_x();
	
	// avoid unnecessary fractional digits in grid tick label,
	// by shifting the grid conveniently into alignment 
	// with integer multiples of grid tick
	double x_ofs = (from_freq_rounded/1000000) 
		- (ffloor(from_freq_rounded / 1000000 / x_tick) * x_tick);
	
	Chart1.SetXYMin(from_freq_rounded / 1000000, -100);
	Chart1.SetRange((to_freq_rounded - from_freq_rounded)/ 1000000, 100);
	Chart1.SetMinUnits(x_ofs,0);
	Chart1.SetMajorUnits(x_tick, 10);
	
	Chart1.SetLabels("MHz","dBFS");
	Chart1.ShowLegend(false);
}

// the returned value is in MHz
double rtlsdr_skyline::chart1_optimal_major_unit_x()
{
	double retval = 1; // default response (1 MHz)
	double decimal_order = 0;
	int chart_horiz_pixels = Chart1.GetPlotWidth();

	if (chart_horiz_pixels < DIV_NO_SMALLER_THAN_PX)
		return retval; // prevent dv_by_zero. Return the default response.
	
	// we could also use "bandwidth" and "num_freqs" and whatnot...
	// but from_freq_rounded and to_freq_rounded are perhaps 
	// the closest to the visual chart.
	double chart_bandwidth = (to_freq_rounded - from_freq_rounded) / 1000000;
	double mhz_per_optimal_div = chart_bandwidth 
		/ (chart_horiz_pixels / DIV_NO_SMALLER_THAN_PX);

	for (decimal_order = 0.001; decimal_order <= 1000; decimal_order *= 10)
	{
		if (decimal_order > mhz_per_optimal_div)
		{
			retval = decimal_order;
			break;
		}
		else if (decimal_order * 2 > mhz_per_optimal_div)
		{
			retval = decimal_order * 2;
			break;
		}
		else if (decimal_order * 5 > mhz_per_optimal_div)
		{
			retval = decimal_order * 5;
			break;
		}
		// else try another decimal order
	}
	
	return retval;
}