Low data plotting resolution depending on graph width

[light]

Bug description

At some graph widths, data gets displayed with low resolution, even though underlying RRA has enough data points.

To Reproduce

I'm using rrdtool python from Debian testing :

rrdtool/testing,now 1.7.2-4+b10
python3-rrdtool/testing,now 1.7.2-4+b10

1. Generate some test data :

#!/usr/bin/env python3

import rrdtool, time

RRD_RRAS = [
   (1   , 4032),
   (2   , 4608),
   (6   , 6144),
   (24  , 4560),
   (72  , 6000),
   (288 , 3660),
   (576 , 5000),
   (2016, 5228),
]

START_TIME = int(time.time()) - 86400*366*10

args = [
   "test.rrd", "--step", "300", '--start', str(START_TIME),
   "DS:x:GAUGE:600:U:U"
]
for rra in RRD_RRAS:
   args.append(f"RRA:AVERAGE:0.1:{rra[0]}:{rra[1]}")
rrdtool.create(args)

for i in range(1, int(86400*366*10/300)):
   t = i * 300
   x = t % (86400*366)
   rrdtool.update("test.rrd", f'{START_TIME + t}:{x}')

2. Generate 2 test graphs :

• rrdtool graph test1.png -w 512 -h 200 --start -10y DEF:x=test.rrd:x:AVERAGE AREA:x#00FF00:x
  
• rrdtool graph test2.png -w 600 -h 200 --start -10y DEF:x=test.rrd:x:AVERAGE AREA:x#00FF00:x
  

At 512 pixels width the graph looks jagged, and zooming reveals that it is drawn with 2 pixel resolution. Whereas at 600px wide, the graph seems relatively smooth.

The 288:3660 RRA has enough data for 300x288x3660 = 10 years (if I'm rounding 1 year = 366 days), so it should get picked, which would provide almost the full 3660 data points for plotting. The next RRA 576:5000, at 10y width, should have 366x10x86400/576/300 = 1830 data points so still enough to plot over 512px smoothly. The last RRA 2016:5228 has 366x10x86400/2016/300 = 522 points for the period, still enough.