We usually depend on Testng for generating the default reports. Apart from that we use XSLT reports as well to generate PIE Charts based on Failed,Passed,Skipped Testcases.
During this process we face lot of issues. Few of the most commonly observed issues are as follows:
[xslt] Unknown file:23:146: Fatal Error! Could not find function: if
[xslt] : Fatal Error! Fatal error during transformation Cause: Fatal error during transformation
In order to resolve this we have to follow the below steps. At every step we will check and see whether issues are resolved or not.
Step 1 :
Make sure the build.xml file contains 'SaxonLiaison' processor added.
Example:<xslt in="${outputdir}/test-output/testng-results.xml" style="${basedir}/testng-results.xsl" out="${outputdir}/testng-xslt/index.html"  processor="SaxonLiaison">

Re-run the build.xml and verify the issue is resolved. If the issue is not resolved goto Step2.

@DataProvider
In order to store the multiple set of input data we use @DataProvider.
We are going to read the xls file which contains input data and store in @DataProvider.
Steps to Achieve :
  a. Store the input data in an xls file.
  b. Read the data from the xls using an third party jar jxl2.6.jar
   Link for Downloading JXL Jar ->http://www.findjar.com/index.x?query=jxl
   jxl stands for -> java excel library
 c. Add the jar to the build path.
 d. Store the data in @Dataprovider
 e. Use the @Dataprovider in the @Test methods.
JExcel API ->http://jexcelapi.sourceforge.net/resources/javadocs/2_6_10/docs/
  WorkBook contains sheets
  Sheets contains rows and column
  Each and every values stored in the rows and columns in a cell.
  Retrieve the contents of the cell using row index and column index and store in an array
Program:
import java.io.File;
import java.io.IOException;
import jxl.Cell;
import jxl.Sheet;
import jxl.Workbook;
import jxl.read.biff.BiffException;
import org.testng.annotations.DataProvider;
import org.testng.annotations.Test;

public class ReadExcelJXL {
@Test(dataProvider="DP")
public void sales_coupon_count(String username,String password)
{

System.out.println("The value of username" + username);
System.out.println("The value of password" + password);

}
@DataProvider(name="DP")
public String[][] readExcel(  )
{
File file = new File("inputData.xls");
String inputData[][] = null;
Workbook w;

try {
w = Workbook.getWorkbook(file);

// Get the first sheet
Sheet sheet = w.getSheet(0);

int colcount=sheet.getColumns();

int rowcount=sheet.getRows();

inputData= new String[rowcount][colcount];

for ( int i=0;i<sheet.getRows();i++)
{
for (int j=0;j<sheet.getColumns();j++)
{
Cell cell=sheet.getCell(j,i);
inputData[i][j]=cell.getContents();
}
}
} catch (BiffException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return inputData;
 }
}
The value of username-->xyz
The value of password-->abc
The value of username-->abc
The value of password-->welcome
The value of username-->ab1123
The value of password-->welcome123
PASSED: sales_coupon_count("xyz", "abc")
PASSED: sales_coupon_count("abc ", "welcome ")
PASSED: sales_coupon_count("ab1123 ", "welcome123")

===============================================
    ReadExcelJXL
    Tests run: 3, Failures: 0, Skips: 0
===============================================


===============================================
DataDriven
Total tests run: 3, Failures: 0, Skips: 0
===============================================
inputData.xls