diff --git a/chart_test.go b/chart_test.go
index dbbb34f..55db3e0 100644
--- a/chart_test.go
+++ b/chart_test.go
@@ -527,6 +527,8 @@ func TestChartE2ELine(t *testing.T) {
 func TestChartE2ELineWithFill(t *testing.T) {
 	assert := assert.New(t)
 
+	logBuffer := new(bytes.Buffer)
+
 	c := Chart{
 		Height: 50,
 		Width:  50,
@@ -550,7 +552,7 @@ func TestChartE2ELineWithFill(t *testing.T) {
 				YValues: LinearRangeWithStep(0, 4, 1),
 			},
 		},
-		Log: NewLogger(),
+		Log: NewLogger(OptLoggerStdout(logBuffer), OptLoggerStderr(logBuffer)),
 	}
 
 	assert.Equal(5, len(c.Series[0].(ContinuousSeries).XValues))
diff --git a/logger.go b/logger.go
index a2fa397..28fc003 100644
--- a/logger.go
+++ b/logger.go
@@ -12,12 +12,16 @@ var (
 )
 
 // NewLogger returns a new logger.
-func NewLogger() Logger {
-	return &StdoutLogger{
+func NewLogger(options ...LoggerOption) Logger {
+	stl := &StdoutLogger{
 		TimeFormat: time.RFC3339Nano,
 		Stdout:     os.Stdout,
 		Stderr:     os.Stderr,
 	}
+	for _, option := range options {
+		option(stl)
+	}
+	return stl
 }
 
 // Logger is a type that implements the logging interface.
@@ -30,8 +34,6 @@ type Logger interface {
 	FatalErr(error)
 	Error(...interface{})
 	Errorf(string, ...interface{})
-	Println(...interface{})
-	Errorln(...interface{})
 }
 
 // Info logs an info message if the logger is set.
@@ -66,6 +68,23 @@ func Debugf(log Logger, format string, arguments ...interface{}) {
 	log.Debugf(format, arguments...)
 }
 
+// LoggerOption mutates a stdout logger.
+type LoggerOption = func(*StdoutLogger)
+
+//OptLoggerStdout sets the Stdout writer.
+func OptLoggerStdout(wr io.Writer) LoggerOption {
+	return func(stl *StdoutLogger) {
+		stl.Stdout = wr
+	}
+}
+
+// OptLoggerStderr sets the Stdout writer.
+func OptLoggerStderr(wr io.Writer) LoggerOption {
+	return func(stl *StdoutLogger) {
+		stl.Stderr = wr
+	}
+}
+
 // StdoutLogger is a basic logger.
 type StdoutLogger struct {
 	TimeFormat string
diff --git a/percent_change_series.go b/percent_change_series.go
new file mode 100644
index 0000000..3767893
--- /dev/null
+++ b/percent_change_series.go
@@ -0,0 +1,89 @@
+package chart
+
+// Interface Assertions.
+var (
+	_ Series                 = (*PercentChangeSeries)(nil)
+	_ FirstValuesProvider    = (*PercentChangeSeries)(nil)
+	_ LastValuesProvider     = (*PercentChangeSeries)(nil)
+	_ ValueFormatterProvider = (*PercentChangeSeries)(nil)
+)
+
+// PercentChangeSeriesSource is a series that
+// can be used with a PercentChangeSeries
+type PercentChangeSeriesSource interface {
+	Series
+	FirstValuesProvider
+	LastValuesProvider
+	ValuesProvider
+	ValueFormatterProvider
+}
+
+// PercentChangeSeries applies a
+// percentage difference function to a given continuous series.
+type PercentChangeSeries struct {
+	Name        string
+	Style       Style
+	YAxis       YAxisType
+	InnerSeries PercentChangeSeriesSource
+}
+
+// GetName returns the name of the time series.
+func (pcs PercentChangeSeries) GetName() string {
+	return pcs.Name
+}
+
+// GetStyle returns the line style.
+func (pcs PercentChangeSeries) GetStyle() Style {
+	return pcs.Style
+}
+
+// Len implements part of Series.
+func (pcs PercentChangeSeries) Len() int {
+	return pcs.InnerSeries.Len()
+}
+
+// GetFirstValues implements FirstValuesProvider.
+func (pcs PercentChangeSeries) GetFirstValues() (x, y float64) {
+	return pcs.InnerSeries.GetFirstValues()
+}
+
+// GetValues gets x, y values at a given index.
+func (pcs PercentChangeSeries) GetValues(index int) (x, y float64) {
+	_, fy := pcs.InnerSeries.GetFirstValues()
+	x0, y0 := pcs.InnerSeries.GetValues(index)
+	x = x0
+	y = PercentDifference(fy, y0)
+	return
+}
+
+// GetValueFormatters returns value formatter defaults for the series.
+func (pcs PercentChangeSeries) GetValueFormatters() (x, y ValueFormatter) {
+	x, _ = pcs.InnerSeries.GetValueFormatters()
+	y = PercentValueFormatter
+	return
+}
+
+// GetYAxis returns which YAxis the series draws on.
+func (pcs PercentChangeSeries) GetYAxis() YAxisType {
+	return pcs.YAxis
+}
+
+// GetLastValues gets the last values.
+func (pcs PercentChangeSeries) GetLastValues() (x, y float64) {
+	_, fy := pcs.InnerSeries.GetFirstValues()
+	x0, y0 := pcs.InnerSeries.GetLastValues()
+	x = x0
+	y = PercentDifference(fy, y0)
+	return
+}
+
+// Render renders the series.
+func (pcs PercentChangeSeries) Render(r Renderer, canvasBox Box, xrange, yrange Range, defaults Style) {
+	style := pcs.Style.InheritFrom(defaults)
+	Draw.LineSeries(r, canvasBox, xrange, yrange, style, pcs)
+}
+
+// Validate validates the series.
+func (pcs PercentChangeSeries) Validate() error {
+	return pcs.InnerSeries.Validate()
+}
diff --git a/percent_change_series_test.go b/percent_change_series_test.go
new file mode 100644
index 0000000..3114bb2
--- /dev/null
+++ b/percent_change_series_test.go
@@ -0,0 +1,35 @@
+package chart
+
+import (
+	"testing"
+
+	"github.com/blend/go-sdk/assert"
+)
+
+func TestPercentageDifferenceSeries(t *testing.T) {
+	assert := assert.New(t)
+
+	cs := ContinuousSeries{
+		XValues: LinearRange(1.0, 10.0),
+		YValues: LinearRange(1.0, 10.0),
+	}
+
+	pcs := PercentChangeSeries{
+		Name:        "Test Series",
+		InnerSeries: cs,
+	}
+
+	assert.Equal("Test Series", pcs.GetName())
+	assert.Equal(10, pcs.Len())
+	x0, y0 := pcs.GetValues(0)
+	assert.Equal(1.0, x0)
+	assert.Equal(0, y0)
+
+	xn, yn := pcs.GetValues(9)
+	assert.Equal(10.0, xn)
+	assert.Equal(9.0, yn)
+
+	xn, yn = pcs.GetLastValues()
+	assert.Equal(10.0, xn)
+	assert.Equal(9.0, yn)
+}