ArgumentError: flag after precision

[yegor256]

ArgumentError: flag after precision
  from SVG/Graph/Graph.rb:671:in `%'
  from SVG/Graph/Graph.rb:671:in `make_datapoint_text'
  from SVG/Graph/Line.rb:260:in `block (2 levels) in draw_data'
  from SVG/Graph/Line.rb:247:in `each_index'
  from SVG/Graph/Line.rb:247:in `block in draw_data'
  from SVG/Graph/Line.rb:201:in `each'
  from SVG/Graph/Line.rb:201:in `draw_data'
  from SVG/Graph/Graph.rb:213:in `burn'

I can't show the code and the data, since this error happens only sometimes. In most other cases everything works just fine.