fix return value

Author: ievgrafov

lib/gnuplotrb/mixins/error_handling.rb

@@ -23,6 +23,7 @@ def check_errors
       end
       message = "Error in previous command (\"#{command}\"): \"#{rest}\""
       fail GnuplotError, message
+      self
     end
 
     private

lib/gnuplotrb/multiplot.rb

@@ -87,6 +87,7 @@ def update_plot!(position = 0, **options)
       return self unless block_given? if options.empty?
       replacement = @plots[position].options!(options)
       yield(replacement) if block_given?
+      self
     end
 
     alias_method :update!, :update_plot!
@@ -110,6 +111,7 @@ def replace_plot(position = 0, plot)
 
     def replace_plot!(position = 0, plot)
       @plots = @plots.set(position, plot)
+      self
     end
 
     alias_method :replace!, :replace_plot!
@@ -137,6 +139,7 @@ def add_plots(*plots)
     def add_plots!(*plots)
       plots.unshift(0) unless plots[0].is_a?(Numeric)
       @plots = @plots.insert(*plots)
+      self
     end
 
     alias_method :add_plot!, :add_plots!
@@ -159,6 +162,7 @@ def remove_plot(position = -1)
 
     def remove_plot!(position = -1)
       @plots = @plots.delete_at(position)
+      self
     end
 
     alias_method :remove!, :remove_plot!

lib/gnuplotrb/plot.rb

@@ -109,6 +109,7 @@ def update_dataset(position = 0, data: nil, **options)
 
     def update_dataset!(position = 0, data: nil, **options)
       @datasets[position].update!(data, options)
+      self
     end
 
     ##
@@ -130,6 +131,7 @@ def replace_dataset(position = 0, dataset)
 
     def replace_dataset!(position = 0, dataset)
       @datasets = @datasets.set(position, dataset_from_any(dataset))
+      self
     end
 
     alias_method :[]=, :replace_dataset!
@@ -162,6 +164,7 @@ def add_datasets(*datasets)
       # first element is position where to add datasets
       datasets.unshift(0) unless datasets[0].is_a?(Numeric)
       @datasets = @datasets.insert(*datasets)
+      self
     end
 
     ##
@@ -181,6 +184,7 @@ def remove_dataset(position = -1)
 
     def remove_dataset!(position = -1)
       @datasets = @datasets.delete_at(position)
+      self
     end
 
     ##