Incorrect plot results with some locales

[alexgorban]

For locales where decimal sign is ',' gnuplot produces incorrect results:

require 'torch'
require 'gnuplot'

locales = {'en_US.UTF-8', 'ru_RU.UTF-8'}

x = torch.linspace(-2*math.pi, 2*math.pi)
y = torch.sin(x)

for i = 1, #locales do
  os.setlocale(locales[i])
  gnuplot.pngfigure('gnuplot-test-' .. locales[i] .. '.png')
  gnuplot.plot(y)
  gnuplot.plotflush()
end

For en_US you will get correct results:

For ru_RU - incorrect: