diff --git a/lib/youplot/backends/unicode_plot.rb b/lib/youplot/backends/unicode_plot.rb
index 7f75c67..01c6277 100644
--- a/lib/youplot/backends/unicode_plot.rb
+++ b/lib/youplot/backends/unicode_plot.rb
@@ -127,11 +127,10 @@ module YouPlot
 
       def plot_xyxy(data, method1, params)
         headers = data.headers
-        series = data.series
+        series2 = data.series
+                     .map { |s| s.map(&:to_f) }
+                     .each_slice(2).to_a
         method2 = get_method2(method1)
-        series.map! { |s| s.map(&:to_f) }
-        series2 = series.each_slice(2).to_a
-        series = nil
         params.name ||= headers[0] if headers
         params.xlim ||= series2.map(&:first).flatten.minmax # why need?
         params.ylim ||= series2.map(&:last).flatten.minmax # why need?