fixed the BADPREC issue when both the maximum and minimum values in a series are 0

[JeffreySE]

Currently, if each value of a series is 0, here are the test code, this PR can fix the issue and avoid if the series's value is always 0.

package main

import (
    "fmt"
    "github.com/guptarohit/asciigraph"
)

func main() {
    data := []float64{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
    graph := asciigraph.Plot(data)

    fmt.Println(graph)
}

Output:

%!(BADPREC)            0.000000 ┼──────────

[coveralls]

Pull Request Test Coverage Report for Build 45

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-20.2%) to 78.279%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asciigraph.go	4	6	66.67%

Totals
Change from base Build 44:	-20.2%
Covered Lines:	191
Relevant Lines:	244

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 45

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-20.2%) to 78.279%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asciigraph.go	4	6	66.67%

Totals
Change from base Build 44:	-20.2%
Covered Lines:	191
Relevant Lines:	244

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 39

• 4 of 6 (66.67%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-20.2%) to 78.279%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
asciigraph.go	4	6	66.67%

Totals
Change from base Build 44:	-20.2%
Covered Lines:	191
Relevant Lines:	244

---

💛 - Coveralls